I have seen them on ebay but never rented from them. They do get good reviews though. Maybe look on Tripadvisor or ask them for some references.
I'd be a little nervous, though I have rented DVC points from DISers which is probably riskier, but I was nervous doing that too. It all worked out fine though.
I would have a "Plan B" in case though. Marriott resorts allow cancellation up to 6pm of day of check in which is very generous imho.
If you have a morning flight you would know long before 6pm if the reservation is valid then cancel the Marriott.