You can also double check the DVC ownership on the comptroller's website. Honestly, about the same risk as going through VRBO.com, etc. You can get a sense of who the person is via phone calls, email exchanges, etc. We are renting in Jan directly through an owner who has been on the board for a long time, doesn't have a ton of posts and only rented only a few times, but has been very kind, open and helpful during the process.
There is so much information available on the web that you can really check out a story to see if it makes sense.
Contracts for rentals are great, but really aren't worth the paper their printed on if someone is going intending to rip you off. More of just a way to document all of the details of the request.
I think you are buying a bit more security going through the middlemen...they would be more likely to sniff out a scammer and rectify any wrongs in the desire to protect their business reputation. We went for the better bargin (fewer $/point), but if the price was the same, I would go with one of the middlemen. I have only read good things about all of them