Any advice would be appreciated!
How about identifying a test user group, giving them an incentive to sign-up/use your marketplace and getting feedback (survey, interview, etc) from them that would help you open up new targeted channels to find & target similar users?
It all depends on your marketplace. Do you have homogenous/heterogeneous supply? Is it open (anyone can signup) or closed? What problem does your marketplace solve for buyers and sellers?
One thing that you must do is reach a critical mass of supply & demand. There are a few ways to “hack” that, outlined here: http://platformed.info/hacking-your-way-to-critical-mass/
Written by Sangeet Paul Choudary, who writes a TON about platforms and marketplaces.