They're collectors. They don't sell much if any of the stuff you see them "buy" on the tv show. It pretty much all goes into their personal collections.
They don't even really buy the stuff themselves as we see it going down. It's pre-bought by the shows producers, and placed back into the piles of crap for them to "find" during the actual filming.
A little bit of looking around of the internet is what brought all this to light.