Yeah, Miata.net does that. I listed a Racing Beat header for the NB '99-'00 strictly for trade, as I didn't feel like paying TEN PERCENT of the agreed sale price to Miata.net. It ended up that I had so many people asking to buy it, that I put it on E-bay, then E-mailed all of the responders to the Miata.net ad and gave them the link to it. I bought the header from E-bay for $201, turned around and sold it for $304(minus shipping). And I didn't have to give Miata.net a cent.
Though I will say Miata.net's classifieds are good for harder to find stuff...I also bought a Jackson Racing highflow cat from an ad in the classifieds for $100, and fully happy...then again I didn't have to pay for the ad, the seller did.