First of all, the power of a marketplace is only unlocked when you have network effects among buyers to retain sellers and when you also have some daunting barriers to competition. I'll assume you know this and you've got a plan to help lock in your network once you gather it.

Even you have tremendous scope of reach like Google did when they launched their Play store, they start by lining up some sellers. An empty storefront will make a buyer bounce and never come back, but if you're persuasive and patient, you can get some sellers to work on something for your store before there is actually a marketplace.

Line up enough different sellers that you figure that at least 66% of those who come to your marketplace will stick around long enough to read about one or more of the products that your marketplace sells. Don't launch until you have them all ready and placed in your store.

Then go out, buy traffic from some PPC or display as networks, and start to sell in your new marketplace!

Good luck, and let me know if you have any further questions.