Check out DVD Talk and Adult DVD Talk. They're not affiliated with each other, although I think they were in the beginning. Both have a forum with a section for buying, selling and trading DVDs.
People on those two sites are bargain hunters. So if you're selling you might not get as much as eBay. But it's a good place to find traders.
DVDTalk is a mainstream site, so there are probably some rules for posting adult material.
http://www.dvdtalk.com/
http://www.adultdvdtalk.com/