As an example....
Take a look at how few listings there are at Visions of the World. Pick a resort and find the number of for-sale listings for that resort. Then compare that to the number of listings at a site like Redweek.com. The more listings a site has, the more traffic will be generated by potential buyers who think they might be able to find their dream timeshare.
Also, at many sites (e.g., Redweek), a buyer can contact the owner directly to make an offer, making it easier for a buyer to consider a purchase. Pretend you are a buyer at VW and try to figure out how to make an offer!