We own DVC at Boardwalk. I have rented my points, but have never rented points from someone else. I rented points to Carl, one of the Disdads. We talked, messaged over Facebook, I booked it for him, and he sent me a check. Some people use an agency. Some people have contracts drawn up. Still, it is all about enforcement, which is not possible.
As a renter, you are at the mercy of the dvc owner. The owner has to book your reservation and send you confirmation # to load into your
MDE. The owner can cancel your reservation at anytime. If you go thru an agency, some require deposits from one or both parties. If the owner cancels the reservation, then the renter gets their deposit back but has no reservation against the owner. If the renter backs out, the owner keeps their points. However, depending on banking window or within 30 days, the points may not be able to be used by the owner, so the could lose them. Sure, you could file a lawsuit, but it's small claims and where does the grieved party file the case or have the other served?
I would rent again to a Disdad, one I know it have met. I wouldn't go thru an agency. But, I rarely have points to rent, so it's only been an option one time.
I bought my dvc contract to stay at the Boardwalk at least once a year thru 2042. I've been able to try most of the other dvc resorts and make two trips a year sometimes.