check out this site. I found it in the new Guitar Center Catalog that just came in. I don't know any more then the address: http://www.instrumentexchange.com
There are a lots of things there to buy, and you can sell your junk (oooooops), I mean things there also.
It is auction style and commission.
Bebop

It has actually been listed on the classified ads & auctions listing at Synth Zone for some time now. I try to keep the links as up to date as possible.