I can say that in the 7 years I've been hanging out on these boards, I only remember a few specific situations where someone actually got cheated out of their money. And by few, I mean less than 5.
The great majority of rentals go very smoothly. And most owners who rent are only doing it to recoup their loan/MF costs, since they can't go themselves that year. It's not a business for these folks, they're just Disney vacation fans like you.
That said, there is simply no way for us to guarantee that all will be well with your rental. It's completely a matter of trust between you and the owner. You won't have any control over the reservation, and you won't be able to confirm it. All you'll have to go by will be the confirmation from Disney that the owner sends you a copy of. The confirmation itself is no guarantee, as the reservation can be cancelled at any time by the owner.
Remember that saying, "There's no such thing as a free lunch?" It applies here. You will save a good deal of money by renting, but there's a tradeoff - the lack of security and comfort you get by booking through Disney. You'll have to decide whether the money you save will be worth the lack of control over the reservation.