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

Thanks for the help both of you, i admire you both as entrepreneurs

Just ask a small fee like ebay does for posting each sale
like (0.5$)

0.5$ after paypal fees would be like, nothing.

^
exactly

THen do 1$ -_-

I wouldn't do any of the above personally...

I would allow people to submit their designs/domains/scripts for free until you have enough traffic/sales and customers to charge a fee. Chances are if people have sold work through your site and are happy with the service they won't mind a small charge of like $1 per sale.

Whatever you do, DO NOT open a site and charge people to list their items when you have no traffic/reputation. Why would anyone honestly pay to list their work/domain on your site? You need to give them the reasons and that would be in the form of previous sales/traffic which you need to spend a good few months working at first.