barter

I'd like to know if there is some web downloadable open source software or free service allowing to manage a barter system.
Here in my city we have a place where periodically people meet to exchange stuff.

A web service where everyone can show the stuff who's going to bring on the programmed meeting, and users can say they're interested in something or another, specifying what they'll offer in exchange for that, and eventually receiving answers like "that will be ok", or "no way man".

Not a system to "place" a real order, just a system to show the stuff you want to give out, and attract potential users interested in it. The "real thing" is always the periodic live meeting.

I wonder if something ready could be used for this, supporting photos, tags, votes, etc.