It works like this:
Everyone can send me STEEM or SBD and a post to promote in the memo. I will then resteem and comment on that post to let everyone know that this person wants to buy votes. I will then track that post to find anyone who upvotes it. When the post is 7 days old I will pay all voters their share of the STEEM as requested by the advertiser and refund any open value.
This is the p2p market and anyone can be a vote seller by simply upvoting one (or more) of the promoted posts.
I hope that answers your question.