This was suggested a while back, but never quite saw the light of day because our primary focus is on serving as a venue for tech support, advice and discussion - rather than commercial exchange. eBay and Amazon do this considerably better than we could ever hope to, so we'll stick with what we're good with.
You'll notice we do have a reputation system with badges for tech support issues, so users can quickly identify users who know what they are talking about and can trust their answers right off.