Buying for 1 trip is probably being rather short sighted. And they'd definitely want to make certain that the reservation they wanted was able to be booked. Welcome Home bookings aren't guaranteed on availability - they'll do what they can for them though. But you can rent a reservation or pay cash or wait until after Dec 16th to go. It can be easy to get caught up in
DVC and that you must own to visit but there are other ways to visit WDW too. Now if you already had a plan to purchase the other resort perhaps but maybe that should be the only resort one buys at.
You could pay cash for the trip and rent out the first points you purchase to "pay" for that cash reservation. So no, if only based on the desire to take a trip before Riviera opens I'd say it's a pretty bad idea to purchase a few points somewhere else.