Another reason to purchase one-time points from Disney rather than renting is that you can't "add points to make a night"...for example, say you're 5 points short for that last night of your vacation....if you buy the one-time points from Disney, you can make up that shortfall and get that last night. However, if you rent from someone else, you'd have to have the someone else reserve that last night for you (for the full point cost of the night) and then link the reservations...
So, say you need five points--in my mind, it just wouldn't be worth $15 (the difference in cost between $15/point for five points from Disney versus $12/point from another owner) to lose control over the last night's reservation, find someone else to make it/link it, etc.
Of course, you could do a transfer and wind up with the ability to combine points for a single night...but then you're using your one "one-time-per-use-year-only" transfer, and the person you're getting the transfer from is also using his/her one-only per use year. This could be difficult with a small number of points.