We could let users sign a bill of sale. That could either be packaged together with the original proof of purchase in a private transaction, or published to avo

We could let users sign a bill of sale. That could either be packaged together with the original proof of purchase in a private transaction, or published to avoid a "double-spending" situation where both the old and new owners claim the item. Or, have the users do the transaction on one of these participating websites and create a new proof-of-purchase and revoke the old one.