Stealing is perfectly possible. You just have to work a bit for it.
1) Turn off all light sources, if possible.
2) Have a character with expertise in sleight of hands and stealth
3) Enter turn based mode
4) Cast guidance or cat's grace on the thief
5) Use a different character to start dialogue with the merchant to ensure they're facing the right way
6) Leave dialogue and check that turn-based mode did not switch itself off
7) Pickpocket and do not get caught (savescum for this part, I guess)
8) Once pickpocket is done, keep turnbased mode on and get the thief out of there. Feel free to dash.
9) Stay in turn based mode for a few turns, but get the thief *away* and into a nice looking shadow off the beaten track. Then hide.
10) Let the merchant go nuts over the pickpocket and accuse random party members. Remember that Shaggy song a while back. "It wasn't me!"
11) Eventually the merchant is going to realise that he can't quite find the thief, quit aggroing over it, and go back to being a merchant. At that point your thief can unhide, stroll right back, and trade like nothing ever happened.
But do keep in mind that you can't quite tell how many vision cones are pointing in a given direction and you have to make a stealth check for each one. And even +4 from Dex and +4 from expertise doesn't quite ensure that you can keep rolling high enough to not get spotted. The game log is terrible in this regard, as it doesn't show you each roll, just "Tav lost condition: Hiding".
Also keep in mind that while the AI cannot see your PC, they do know where they last saw your PC, and they generally will try to go to that spot and see if they can spot you. And if they have darkvision and you're like 8 feet away from the spot, then you're not exactly covered by the darkness.