The new interface is very buggy. Double-clicking trades something, but often leaves the trade button lit up, which, if clicked, then either takes more money from you, or takes more money from them, or returns the last item you traded - and doing so causes a reputation shift; positive, as though you'd given them free money, and negative as though they'd lost that much money.
Items that get caught up in this buggy exchange disappear form both sides, but then reappear after you trade your next time, usually - often acting as though you hadn't sold them in the first place, which lines up with the rep negative for 'you stole money from me', sort of.
It's very buggy, but difficult to report reliably, since it can be done on any save file in any shop window.
