Actually, I thought the owner got half the money up front and the other half at the start of the trip.
You have to remember that with purchasing a reservation from a DVC member, it's between you and the member. Disney will not get involved in any way. You won't be able to call about the reservation, to add Magical Express, to add dining. You'll have to pay in full to add the dining plan when you add it to the reservation. There is no possiblity of a surprise upgrade unless the villa you booked is taken out of service for maintenance. And then you might even be moved to another DVC resort to a villa of the same size and category.