A tiny bit of SPOILER alert, but not that much really.
A ) Leveling FAST as a Thief right from the start of the game
Start a new game. Go to Ragpicker's Square, talk to Ratbone and become a Thief. Put all your starting points into Stealth. Even with 9 dex the 40 points into stealth you'd have put will be enough.
Next, acquire a Green Steel Knife, Green Steel Dagger, or any +1 or better enchanted weapon for that matter. Dagger is best (2-5 damage, compared to knife's 1-4). You can buy it at Hive Market at one of the merchant for a very paltry sum.
Now you can kill Black Abishai (which respawn as soon as you leave/reenter the area and are worth 7000 XP) and you can Hide in Shadows quite reliably. So, you walk up to an Abishai, you quicksave the game, Hide in Shadows (best to use a convenient hotkey), attack the Abishai. After an attack animation is executed but best before the Abishai can respond, you simply run away from it till you vanish from it's sight, so you can Hide in Shadows again. If you fail a Hide in Shadows, just try again - the cooldown is nonexistant unlike, for example, Baldur's Gate or Icewind Dale. It's quite exploitative.
This way you're either not getting hit at all, or getting hit very rarely, and kill an Abishai relatively quickly even with 9 Str. Especially once your Backstab multiplier improves.
Take notice that if you don't want to Specialize as a thief, do not level up into level 7 or 12 thief without attaining these levels with a class you do want to specialize in first.
If you want to have maximum Base Pick Pockets skill (100) by level 6 (and so without Thief specialization) you have to put ONLY 22 points into Stealth and the remaining 18 into Pick Pockets.
B ) Maxing out Pick Pockets
You can use it to pickpocket random NPCs, but also to steal from shops. Stealing from shops works like this, as far as i know - a shop that can be stolen from has a Steal Difficulty which is subtracted from a character's Pick Pockets skill, and the remaining percentage is the chance of a successful steal. What item you steal, how "big" is it or how valuable does not matter. And so with Old Mebbeth's (Ragpicker's Square) 30% Steal Difficulty you need 130% or more of Pick Pockets skill to have, it seems, a perfect 100% success chance on every steal-from-shop attempt. You can find information on what shops have what difficulty, which ones can be stolen from and what can be stolen right here: "http://torment.wikia.com/wiki/Store_(PS:T)
" (at least at the time of writing this article, otherwise look it up yourself)
Initially, as you become a Thief for the first time, you have an automatic 22% in Pick Pockets. This is equivalent of 22 points on level up screen. This is your "base" pick pockets, after which DEX and all other modifiers get applied.
There are TWO ways to increase Base Pick Pockets skill - assigning points from Thief level ups or getting Permanent Bonuses from special sources.
To maximize your Pick Pockets, first assign points from level ups until you can't any more - this will put the Base PP at 100, which is the maximum you can get from level ups. (It might not necessarily show a "100" on the level up screen if your Dexterity is below 13, which gives you a negative DEX modifier to your Thief skills. That's okay, though. If you'll improve your DEX later on, the modifiers will change and there'll be no harm done). It's best to do that before going after any permanent bonuses from special sources, because it'll allow you to go over 100 Base PP - otherwise you'll just save a couple thieving points at level up, which is rather unwise since you can level up indefinitely and thief points from level ups are really not that scarce anyhow.
In case you skipped Tip A, if you want to max out your Base PP by level 6 (to avoid Thief Specialization) you can't assign any more than 22 points into thief skills other than PP.
Permanent Bonuses to Pick Pockets thief skill:
1) "Fleece" NPC, NW Hive (before Ragpicker's Square) - you need to spot his thieving attempt after you chit-chat and say goodbye (about 13+ Wisdom or PP above 10%) and you need at least 16 Wisdom to get the maximum bonus (Bait and observe his attempt carefully). = +5%
2) "Ash-Mantle" NPC, Hive Marketplace - similarly to above. You also need 16 Wisdom. = +5%
3) "Drunk Harlot" NPC, Smouldering Corpse Bar area - similarly to above. You also need 16 Wisdom. = +5%
4) "Yellow-Fingers" NPC, Ragpicker's Square - avoid fighting with him until you get the bonus. You NEED at least 12 Intelligence (Wisdom or Pick Pockets skill won't work) to spot his attempt. Wisdom doesn't affect this bonus. = +1%
5) "Annah" NPC, joins you after a bit into the Main Quest - after she joins you simply talk to her and ask her about improving your thieving->Pick Pockets skill. Wisdom doesn't affect this bonus. = +3%
6) "Tattoo of Silent Coins", find a certain item in the Catacombs that can unlock new tattoos at Fell's Tattoo Parlor (you need high Intelligence or somebody to translate for you) - simply "use" the tattoo, which you should do because it does not delete the tattoo itself or make it lose its effect. = +3%
Amounting to 5+5+5+1+3+3=22%
With 100% Pick Pockets from thief level ups, then all the bonuses, you get 122% Pick Pockets.
Now, here's the thing. It seems your Absolute Maximum of Pick Pockets skill is precisely 170%, which comes from 125% Pick Pockets + 45% DEX Modifier from 25(max) Dexterity.
With 122% Pick Pockets (before DEXmod) even if you put on both Thief Specialization tattoos and Tattoo of the Silent Coins all it'll do is make your 122% Pick Pockets into 125% Pick Pockets. You need 25 Dexterity if you want the absolute maximum.
Therefore, after getting all the Permanent Bonuses to reach 122% Pick Pockets, simply wear Tattoo of Silent Coins (if you don't have/don't intend to have any Thief Specialization tattoos, which have other effects in addition to increasing your Pick Pockets by 5%). This gives you 125% Pick Pockets. If you want it increased further, you got to increase your Dexterity - in whatever way. 9 Dexterity gives a -15% DEXmod to Pick Pockets, 13/14/15 have a precisely 0% DEXmod on all Thief skills. As it increases, so do the DEXmods - maxing out at 25 Dexterity which, as said above, gives 45% to Pick Pockets alone.
In case you did not know yet, the second highest Steal Difficulty ever is a certain Apothecary, sitting at 70%. Meaning, if you have 170% Pick Pockets whenever you switch into your Thief class, you can steal from everybody at 100% success, except one, certain, "curious" shop which has 95% Steal Difficulty.
And in case it's not obvious, with high enough Pick Pockets, you will probably never pay a single coin for any healing item ever again.
C ) You CAN sell stolen items. (Provided they're a stackable item, like a Charm or a Scroll)
Yes, you can! It's seems to be a bug, though, in which case some kind, unabusive soul perhaps ought to report this. This is a downright exploit as unlike Baldur's Gate 2 in PS:T there are no fences that buy stolen items. (And yes, in Baldur's Gate 2, you just find a fence, sell him a valuable item/items, steal them back and "repeat until false". Also, I haven't checked if this particular exploit works in other Infinity Engine games)
However, as mentioned, I've found a way, though it works only for stackable items. It turns out a "Stolen Item" flag gets checked only on the "first item of the slot". As in, if you have 1 stolen Clot Charm, buy 1, then try to sell it - no bueno.
But if you first buy 1 Clot Charm, then steal 98 Clot Charms, then try to sell a WHOLE STACK (99) you will sell it successfully. It brings to mind a type of scam where you cut paper into a bunch of bill-shaped pieces, put a real money bill on top of it, then wrap it so tightly that, unless looked closely, will resemble as if it was a giant wad of bills, not just one bill and a bunch of paper clips.
Either way, your money problems don't exist any more after this. Sure, you could just use a console command, a mod, or cheat engine to get it done - but it's all different roads to the same outcome.
Best way to do it is to find a merchant with unlimited stock of a high-value item like a Cockroach Charm, Bone Charm, Blood Fly Charm or even just Heart Charm. Magic scrolls of higher levels may be even better, though you may need to invest in purchasing a stack then bringing it to somebody you can steal from. You buy 1, steal 98, but don't sell it back. If the merchant has an unlimited stock you'll get vastly inferior rates.
Instead, find a merchant that doesn't have any of the particular item but is willing to buy it, paying way more than he would have if he did have the item. For example, two merchants right next to each other in Hive Market. The first has an unlimited stock of Heart Charms. The second one, which ordinarily sells weapons, happens to buy charms as well. So sell him a whole stack, then buy 1 back, steal 98, sell 99...and so on.
Or at Old Mebbeth in Ragpicker's Square just buy 1 Cockroach Charm, steal the remaining 2, sell 3, buy 1, steal 2...you get the idea.
Just be warned that if you accidentally steal a whole stack you will have to make another "fresh" one to reset the flag. This is why it's worth it to find a merchant with an unlimited stock first - or just quicksave every now and then.
Easiest route is to get a bunch of Heart Charms or Bone Charms (at the start of the game) and later on 5 scrolls of Power Word: Blind from Civic Festhall (there are 3 NPCs selling them here and grinding money with them is absolute best approach - in turnabout of 5 scrolls which is 1 stack you get over 20k coins) and then bring them to Giscorl in Hive Marketplace, because he buys both Charms and Scrolls and his Steal Difficulty is the lowest in the game - only 10%, so 110% Pick Pockets is enough. Also, you can position the stealable item at the 6th slot of Giscorl, so you have it the closest to the Buy/Sell/Steal buttons
Using Dirty Rat charms, Dexterity tattoos and changing into a Thief at Ratbone just for a moment before switching back is the very base minimum to stockpile jink-jink. With her innate 18 Dex you can even get Annah, in but a couple of levels, to the 110% threshold.