There are many such sites coming up every day and being announced here in bitcointalk.
To get users, particularly buyers you will need a continuous campaign across the forum by means of bounties and signature campaign, without which the site cannot self sustain.
The reason why the Signature Campaign was made, to make the site more popular. Well, it is the obvious way to advertise a new site. But it don't need to be a continuous one, if you think you can stop the campaign, it can be stopped. But if you really need more buyers and users for your site or service, I think you will be needing to keep it a little longer.