Feature request: Ability to reverse merchant transactions

I would love an "undo" button of some sort in the merchant interface. This would be handy for when you accidentally sell something you wanted to keep, inadvertantly purchase the wrong item, or had "buyer's remorse" after finding out that the expensive armor you just bought doesn't look good on your character. A simple "Reverse last transaction" button would be all that's needed.

So would this go across sessions?

No, once you exit the merchant screen, the undo queue would be cleared.