TwtMarket aims to connect supply and demand in the Twitterverse. As a website it leaves much to be desired but it isn’t really made to be a service itself. More like a guide. (Thanks to Chris Brogan for finding this one)
You need to follow and send @twtmarket your tweets to begin, but then the process is simple. By marking what you #need with a #hashtag, as in their example
you get back a message from Twtmarket that someone has what you need. The cool part is that expands beyond actual goods and into jobs and anything else you can imagine. Finally you can also recall any time by sending an #offer nothing.
There is even some advanced features to block specific users, report spam and search.