From personal experience, I've worked with
DVCSales,
DVCResaleMarket, and
Fidelity. They've all served their purpose (purchasing/selling), but I have to say that out of all our experiences,
DVCSales has been:
1) The most efficient (offer till closing was just over 1 month)
2) The most courteous/responsive (Mark and Lori never shy away from the occasional email/phone call and the other two made me feel as if I were pestering them after the offer was sent for ROFR. In addition, their POC at Mason Title, Cammy, is also exceptionally courteous/responsive)
3) The most transparent (Their website requires back and forth negotiations directly between you and the seller through their online system, which cuts out the need for the broker to play the telephone game in between buyer and seller. This also ties into efficiency, point #1).
Hope this helps, and good luck! =)