This idea is for specific rooms at spawn designed for trading. That way, you don't have that feral run by you and pick up the db you just threw down. I'm not sure you need anything more than just a room, or maybe a like a small public shop chest that people can push a button to get a sign for to have one transaction (the info for the transaction would also be tracked). 

I think more useful would be a specific chat command that initiates a 1v1 like mini chat room. Maybe just /trade [name] and then /untrade to get out of it. All messages in between would be sent directly to that person, and they would receive a prompt to do either /trade [name] or /tradeaccept. 

I was also thinking about this and think it could even be realized with simple redstone mechanism, but yes a room for this matter would be beneficial for some people :)