What does VOW do to engage sellers?
It delivers the Big Picture, showing Sold, Pending and Contingent listings (in most MLS areas)
The seller can narrow their search criteria and have the system email them regularly so they can keep up-to-date on the sales activity in their area.
So just as buyers return to your site on a regular basis, as they have a reason to do so, you will then have a nice collection of sellers, in varying degrees of readiness to move forward.
And we probably do not need to tell you that buyers, once they become more serious, will want this advanced information as well.