Seems this could have easily been avoided if people used their PGP keys and signed contracts that detailed the scope of the transaction.
Even GPG aside (or with GPG), a simple way to detect this would be to have the buyer/seller verify their agreement over a simple email exchange.
The (real) buyer should send an email to the (real) seller's paypal payment address, including their receiving bitcoin address as a confirmation before actually sending the funds to the seller's paypal. The (real) seller would then see that the bitcoin receiving addresses don't match. Actually, email aside, I think you can send notes along with your paypal payment to the seller.