There have been a couple of problems over the years. A couple of cases where a non-owner was scamming, and another case where the owner sold the contract after the rental reservation was made. The renters were made whole in at least one of the scams (after legal pursuit of the scammer) and in the sold contract case. In another scam case the renter chose not to pursue legal action against the scammer. Considering the number of rental transactions on our boards.
In all the years we've had the Rent/Trade board, there have only been a handful of problems with a transaction.
Rental transactions are largely a matter of trust on the part of both parties. There are things you can do to help prottect yourself, includig getting full contact info on the owner (name, address, phone, email, etc.), agree to a contract that is fair to the interest of both parties, follow your "gut" feeling...if it feels wrong, don't send $$. But remember that these are private transactions between two individuals, Disney/
DVC will not become involved, nor will the DIS.
Here are some threads you may find helpful:
DVC Point Rental Primer
and
Considering Renting from a DVC Member?
In the end, only you can decide if renting is beyond your personal comfort level. Renting isn't for everyone, and those that need a feeling of full security would be better off renting a cash reservation directly through Disney.