I can't find a script or stat where the information is stored so it can be reset.

Achievement stats are stored in your account on the steam server. They are added via script, but cannot be undone or reset.

So there is no possible way to change my stat for "Barter Items Traded" back to below 10000 in order to gain that achievement?

Guess I'll have to wait for the next play-through or dig out an old save game.

Try looking under ActorData/Challenge.