I'd imagine WBAC is a bit of a gamble on their part.
Like any second hand dealer of any sort you'll find some golden nuggets and you'll also get a load of dross. Its how you handle the dross thats the key to success because the dross will drag you down.
I'd imagine they triage their purchases into different paths to market, but you still need an effective way of dumping the unsellable or you'll rapidly end up with too much capital held in dead stock. Better you sell the dross quickly at cost (or even cost minus) than keep it.
The secret to their obvious success must be in getting those numbers balanced somehow. I'd imagine having good cost controls on storage, transport and auction fees will be one key element of the operation.