Internet traffic

I would say both, you need to have a balance between them and you need both of them to have a thriving site.

I am running into a similar issue with a business that I run.

In reality, you have no end user if you don't have any listings. On the flip-side, you'll have no listings without end users (ie. why would I list on your site?).

One thing you could do is build up a list of leads for both the listers and the end users (get their emails!). Once you have a decent balance of both, release the finished website and contact your leads letting them know that you are up and running.

One thing you want to make sure is that you are up front with your listers and end users. Let them know that this is something that is just getting started and something that you are currently in the process of building. The last thing you want to do is invite people to this "Massive Party" only to have them show up and no one is there.

When you start, you should focus on listers and then on ends users.