How do i start a membership site?

How do i start a membership site?

Hi everyone, i have a website in a niche and its on first page on google. I am thinking of turning it into a membership site which will be free for buyers to browse, but sellers will have to pay a monthly fee to sell their products.

The hard part is how do i accomplish this with nothing for sale on the site yet?? Somehow i need items to be for sale so when the buyers come along theres actually somthing for sale.....