I think instances of bad rentals are VERY rare. I remember one situation here that I believe eneded up being an honest mistake. If memory serves, it had to do with someone who rented points right before he sold his contract. When a contract changes hands, all existing reservations are cancelled by
DVC.
Other than that, as long as you are careful in who you deal with, you shouldn't have any problems. Four things I would recommend:
1. If you're looking for a rental partner here, choose someone with a high post count. You can't get 1000+ posts to your credit overnight, and it seems pretty unlikely that someone would roam these boards for months and then start scamming folks out of their hard-earned $$$.
2. Actually speak to the person on the phone. Don't simply rely on email as your only form of communications.
3. Have a written contract that explains all the terms of the deal including when payments are due and what cancellation provisions are allowed (if any.)
4. Ask for references from prior rentals. We rented a couple of times before becoming members ourselves. And, after each transaction (both of which went perfectly) I always told the renters that I'd be happy to serve as a reference if needed.
Good luck, and welcome to The DIS!