It can be significantly cheaper, but there is a risk involved. This will be a private transaction between you and the owner/member. Disney will not be involved, and will not step in to help if there are any problems. You will not be able to check on the reservation yourself, only members can call MS. If you need to cancel or change the reservation, you may lose your money. Many owner/members have "no refund" policies for cancellations, especially if the points are close to expiring or would go into a holding account.
If you find some one you can trust, and feel comfortable with (and comfortable with not controlling your reservation) than renting can be a fantastic way to get a bigger/better room at a great price.