Let's say that I sell parts for a 1962 Volvo P1800. I'm on eBay, and people buy through eBay and pay through it, but I remain as the seller. If suddenly, however, I have a complete car to sell in parts, that may ring a bell. Potentially, there is something....
You act, then, as a conglomerate, as a counter to unite buyers and sellers. You take a commission on each sale, so the advertised price you're talking about is the advertised price of the seller. If he bought a ticket for $50 and sells it for $100, you take 10% on the $100.