Listing Fees

An idea relevent to that would be to make two lists:

Seller Audience & Buyer Audience

Weigh the lists together and see if they balance, well ideally you'd pretty much want to have more potential buyers than sellers, as not everybody will be buying. Try and analyse where you're advertising, how you're advertising and allocate a budget before hand to try and keep some structure and reason to how you're going to grow your site.

Of course I don't have to say, this is only one of the many slices of the web development pie