I think that you take a risk in any transaction with an individual owner. They are often just people who have more timeshare weeks than they can use, not people that do a lot of renting and have sophisticated systems or are even on paypal. More and more people are on paypal these days, though.
That being said, though, I've done about six transactions on Redweek and had a great experience on all six. All my Redweek rentals were sending 1/2 of the money up front via check, getting the certificate in my name and checking with the resort to make sure they had a reservation for me for my dates, and sending the rest 30 days before my date. I always get the address and phone number of the owner too and talk to them on the phone. I usually call the resort again right before going to make sure there is still a reservation in my name. In reality, the owner, could at any time change/cancel the reservation and leave me cold, but that has never happened to me to date. My previous good experiences, the money I can save, and the money that I've saved through the years has made me willing to take this risk.
As for assurances, though, on all the rentals that I've ever done there really weren't any. They people sounded really honest and nice on the phone, but I'm sure a con artist could sound that way too. I've done lots of rentals from TUG too that worked the same way.