It's being sold in the back of a truck, taken to a pawn shop, or some other direct channel. Most thieves know the police are constantly watching all the online sites and the rest, so they are selling it to somebody for $50 who will sell it to somebody else for $75, etc.

If it was just hard drives, it could be they are teamed up with a repair facility who is buying them, they pay $20 or so, and when somebody brings in a machine needing a drive or an upgrade, they install your freshly formatted drive into their machine.

If they were anything but stupid kids from down the street, you will probably never see your stuff listed on the normal buying sites.