We need help In Website to Buy as well as Sell Digital Files

I want a website created where users are able to buy and sell digital files. The users will have to all have usernames as well as they must be able to post a file which will be available to any other user to purchase, having a title for the post, price, preview photo, a description box, ect. Fundamentally it will be a marketplace for digital files. A user must be able to search for specific files both by similar title and by file type. I would like for all transactions to happen either on the site or through PayPal. When a file is sold to another user I would like 10% of the commission. The site must also use security images when users upload or download files to prevent spam. Afterward a file has been purchased an email desires to be sent to the user and the link needs to expire within a few days. I want to be able to regulate (delete) transactions whenever necessary. As well I want to be able to limit what file types are allowed on the site (ex.pdf allowed but .exe not allowed).

Desired Skills are MySQL Administration, HTML and PHP