Actually it depends on what the goal is for the site. Right now the business is still in its infancy, and like any new business the focus will be on cash flow, so it needs as many paying advertisers as possible.
As the site matures, more people use it, more services are added, and it becomes profitable, they could afford to vet vendors and market to buyers that way. Vendors who are permitted on the site could charge premium prices based on that reputation. You're looking at years to get to that point though.