Current Behavior: The multiclick window maximums only update upon purchase. This results in 2 distinct symptoms.
1:The first time you multibuy you are limited to the amount the character who speaks to the vendor can carry (i.e. if you speak with slot 1, then switch to slot 2 to purchase, the UI uses slot 1's limits
2:On subsequent multibuys you are limited by the inventory space of the most recent purchaser. This is most noticeable when mass purchasing ammunition. For every switch you need to purchase a single stack, then you can multibuy the rest.
Expected Behavior: The multiclick window updates upon character switch to the maximum that character can carry
Obviously not a huge issue, and it has a really simple workaround but I figured I'd file the report since I noticed it. The problem can either prevent you from multibuying up to a character's limit, or if the previous purchaser had more space, allow you to select (but not actually purchase) more than the chosen character can carry.