We have always bought DVC new, direct, so I spend a lot of time justifying it to myself. I have heard that Consumer Reports estimated a 13 year break-even point without financing, as compared to renting a similar property. I calculated ours takes about 15 years to break-even including financing, based on what points could have been rented for.
That sounds like a long time to pay off, but viewed the other way, there are 35 years of "profit". Our Boardwalk is now "profitable" and Animal Kingdom is close behind. Buying resale costs less, but you will also have less years after breaking-even, which I figure reduces some of the benefit of after-market.
I can see someone buying resale for a specific property, like Boardwalk or Beach Club because they really enjoy Epcot festivals. The advice most give is buy enough points (we usually wish we had more), and buy where you want to stay. This is more important as it seems harder and harder to book outside your home resort at the 7 month window.