You have a chicken-and-egg (mutual baiting) problem, not a marketing problem per se. And in building your platform you will run into other generic platform problems after solving this one e.g. the ghost town problem.

The good news is that there are a number of well-defined solutions to these problems; I'm aware of about 8 solutions to the chicken-and-egg problem. I'll provide some quick thoughts based on the little I can randomly glean/guess from your question but I would need to know much more about your specific platform to make a well thought-out suggestion:

* You mention "user side". So this may be connecting users of some kind to merchants of some kind?? A common approach in such scenarios is the "marquee merchant" approach where you get one respected merchant (like an anchor tenant in retail shops) that can be the magnet to get sufficient users on board, and then you can use the growing user base to get more merchants. Obviously you have to be willing to make certain special concessions to the marquee merchant(s).

* You mention that you have traffic. Another common approach is to focus on the side that is easier to get. So the question is: where is most of your traffic coming from? Which kind of participant (the term "user" is vague - you should be talking about "producers" and "consumers" in a platform) is predominantly coming in?
-- If you are getting mainly consumer-side participants then maybe you can create supply-side proxies like Yelp did.
-- If you are getting mainly supply-side participants then maybe you can figure out standalone value for the producers, like Opentable did.

The more info you make available the easier it will be to help. Good luck anyway.