I second Dave's as a very reliable, reputable company. I have used him to rent DVC, and when my request was not available, had my money promptly refunded.
I'm talking about paying $3,000, the DVC Member cancels the reservation - that sort of guarantee. No place will refund that $3,000 as they have already paid the member most of the fee. Your only option is suing the Member using your rental contract.
I have be using Paul Little - DVC Rental Store. He is awesome. The guy answers emails at 1AM.