I recently sold a contract through www.dvcresalemarket.com They were great to work with, from helping me choose a listing price to keeping me updated at every step in the selling process. I would use them again without hesitation. I chose them because they are one of only a couple of brokers with a sales commission below 10%. They are at 8.5% now but were at 8% when I listed with them.
I have purchased through The Timeshare Store and they are great too. You will find lots of good reports on them here. They are also a board sponsor.