to expand on this idea:
perhaps a feature where you can mark a character as a "trade," insert the user's username, and select the character from their characters that you're trading for? (similar to how you add a character someone else owns to an image)
this could of course backfire, so i'd also request an option to decline the trade and neither characters will be transferred! they will only be transferred if both parties give the "ok" on the trade.
or as an alternate option, the other user selects their own character and must okay it as their half of the trade, then the 1st user must accept the trade agreement again?