Whether you just have a few extra parts laying around from an old project, or selling car parts is your full-time gig; sign up to receive notifications for the parts you have on-hand. Sit back and wait for the site to email you relevant part requests from other wrenchers.
Sign-up to receive part requests for a specific Year, Make and Model. Subscribe for the entire marque, or be as specific as a fuel pump for a Audi B5 S4. No need to follow the forums and Facebook or post online / eBay. Let the most relevant requests come to you.