I've done about five rentals from five different owners on that site. All worked well for us. Not sure I can give advice on how not to be scammed (I really don't know if anything is entirely foolproof and for people who are really concerned about that, you might want to just pay more and do retail), but I'll just let you know what we've done before and how it worked. I was never in any of my five transactions concerned about the possibility of being scammed. But I'm sure if someone was a really good con artist and a scammer I could have been had.
Anyway
1) sign on as a guest and find property we want to rent
2) join redweek.com and email owner.
3) exchange emails, phone number, and I usually get the owners home address too, agree upon price, week, and terms and set up time to talk on the phone
4) usually on the phone the people have just been so nice and seemed really legit and that gave me a huge sense of comfort. Often they offer to give references too if you want to email others who have rented from them. Also, these days many times you can do your transactions through PayPal which is another layer of security. If you are concerned about getting scammed, maybe you just want to look at renters who take paypal. If you understand paypal and can help owners out, you can even explain to the owner how to join paypal so you can pay them through that. My mom has done this before. I'm afraid I personally only know how to sign up as a user on the purchasing end // haven't taken the time to find out how the other side works.
5) Pay the owner 1/2 the money
6) Owner sends you rental certificate (usually it's something like an RCI guest certificate)
7) I also usually call the resort to make sure they have a condo listed in my name for my week
8) Pay the owner the rest of the money 30 days before the stay
9) Week before -- owner usually sends a nice email -- wishing us a wonderful stay
10) Week before -- I do a quick call to the resort and make sure there is a condo in my name for my week
11) Have a great vacation, stay in a great condo at a wonderful resort for about 1/3 the price of retail
12) Once home, email the owner -- tell them how much you loved the condo and to feel free to use you as a future reference as they were so great to deal with, add them to contacts list as a possible future person to rent from if you think you'd like to rent at the same resort again sometime.