All this talk of forms, signatures, etc are of little to no value when dealing with PayPal for intangible goods, such as domain names.
PayPal similar to checks, ACHs (e-checks), credit cards, etc is REVOCABLE ...
If it's something expensive / important, require a non-revocable payment method, such as bank wire (often confused with ACH, which is revocable), western union / moneygram, cash, or escrow.
On a related topic, the suggestions of emptying a PayPal account, withdrawing right away, etc are poor advice - occasionally tactics like that work, but more likely PayPal will draw the funds from an attached credit card, checking account, etc ... and if that fails, they will potentially sell the debt to bill collectors, file a lawsuit, etc - why deal with all those hassles when there's a better way ... requiring NON-recovable payment to begin with.
With all that said, one should know who they are dealing with, especially if the amounts involved are anything more than trivial... and again, require a non-revocable payment method / escrow.
Ron