As a general rule, you want to focus on the merchants first. We can assume that the merchants will always be merchants, and thus will always be interested in serving new customers whenever they make themselves known. Merchants don't necessarily need to see immediate customers, they just need to be convinced that the opportunity to gain customers truly exists. Unless you are charging some kind of fee upfront (usually not recommended), then I believe at least some of your merchant prospects will be happy to sign up and wait.
There is also an opportunity to sell exclusivity (maybe a waiting list?), depending on your business model, and the type of marketplace your building. If you can make it cool for merchants to be first then you have something special. You can also make your early merchants part of your growth strategy if they're adequately incentivized to promote their presence on your platform to their customers and prospects.
On the other hand, the end users or customers, may only have a need at a specific point in time, so when they come knocking, somebody better be there to open the door.
Hope that helps. Good luck with your launch!