Oh, I agree completely. They could report it stolen after the sale. But at a carrier store, the employees could (and I stress could, but not necessarily will) verify the selling party's identity, verify they are the rightful owner of the phone (or at least the main account holder), that there's no remaining contract on the phone, and that the phone has been removed from the sellers account.
Now, I don't necessarily know that they WILL do all this, since a private sale between two third parties does not in any way benefit them (thus no incentive to perform those tasks), but at least it's a place to start.