Once you finish the Slavers line of quests where Hendak kills Lehtinan and takes over the ownership of the Copper Coronet any items you sell to him can be bought back at a significant discount (including of course any items that he has on hand).
This pays off big when you loot a Wand of Frost for example with only 8 charges. Normally, you might sell it to Ribald for perhaps 300gp and the store recharges the wand right up to the full or maximum of 100 charges but the buyback price is much much greater around 6500gp,.
If you sell it to Hendak the store recharges the wand to full or maximum but the buyback price is only 4000gp,! That's 2500gp in savings or 40% off,!
You may even wish to sell all your extra or unused items at the Copper Coronet
Hendak's wholesale purchase price appears to be the same as the rest of the merchants
make sure and get your Reputation and Charisma up as high as possible before any expensive purchases int he game