Private sales do happen. The advantage to the seller is that they have more in pocket. The buyer can frequently get a slightly better deal also but with the ROFR, the savings may not be that great. The yahoo dvcrent is one place in addition to TUG. There are also many net locations where one can list for free including
www.timeshares.com and the FSBO site in FL but I don't have the URL at the office. I've seen them in the news paper as well, actually bought one of mine listed in the USA Today though it was listed with a resale agent and we ended up closing through a firm in Miami who I would not use again.
If you're thinking you can save a lot privately, that's not likely unless you can cook up a scam to make
DVC think it's lower than you're actually paying. Still, one can save some on resell over buying from DVC, maybe $2000 or so on 200 points depending on specifics, points availability, closing costs and the like. Just remember that you pay no closing with DVC but will usually pay about $450 resell.