I can't speak for those that are the usual DVC sellers but in general it's a minimum with a percentage. Usually a minimum in the range of $995-1500 and a percent of 10-15%, which ever is greater. I know of at least one DVC reseller that charges 12% but has been known to drop it to 10%. This only DIRECTLY affects the seller.
Up to a point. I'd guess anything less than $1000 is not worth them even fooling with. And i've found most of them only willing to negotiate up to a point. Usually it's when they're higher than the others anyway, you're looking at multiple sales or there's a problem with the sale where money is involved.