Has been posted many times, but trust is the biggest element. You should receive the e-mail with the link to the reservation. You can check this at any time. You can call the resort prior to leaving to confirm the reservation. Most important, ask the owner for references. Those who are legitimate will have no problem providing those. Also look at the owner's history on the DisBoards - date joined, # of posts, look at the history of the owner's posts.
None of the above guarantee a non-scam, but will significantly increase your confidence level. While renting scams occur, they are rare.
If you aren't comfortable renting from an individual, there are a number of brokers you can rent from, you'll just pay a little more going through them.
scamming is rare, but if renting directly from individual, look for someone with a history as a renter.
the biggest issue is "do you understand what you are getting?" renting DVC can be a big savings, but you almost always lose any cancellation refund options and there may be tradeoffs (like very limited housekeeping and sofabeds instead of real beds) that work great for some but are off-putting to others.