Transaction verifications on Paypal by transaction ID's. That it can be verified via a page on PP that transaction ID X happened on day Y, for the amount of Z, between A and B (emails), A being sender, B being receiver. Either by a direct page (think thats unadvisable for PP) or the owner of the paypal addresses allowing a 3rd account to verify wether a transaction did or didn't happen. Scammer A didn't pay to B, does some photoshopping and posts pictures of "payment" to B. As the Transaction ID is known, as well as the emails, it should be easy to verify a payment did occur or not on a date. I purposedly didn't ask for steam stuff, for that won't be given anyways by Valve. But the above would be feasible for either party could be give permission to review a single transaction. And if it doesn't exist, they could still "allow" the check on the ID vs their account... which would show that such transaction didn't occur.