I don't see how the rip and flippers do it with that one guy listing 2,400 sketches right out the gate (and I guess there are one or more others who do that too). Seems like that will destroy resale value.
As a buyer, it's great, but as a seller that would suck.
My sketch collection: http://sketchcollectors.com/gallery.php?user_id=727
Claim to fame: Coined the term BODA