How to use paypal on my website?

I want to make a website where people buy and sell files to each other such as pictures. It'd be like ebay for virtual items. I wouldn't make any revenue, it'd just be instant transfer from one person to another. what would I need to do to make it happen?

Learn to integrate PayPal into your own website. (http://web2coders.jrcw3.com, easy paypal integration script).
Once you understand how it works, establish a database of your users.  Each one must have a paypal account.
THEN, payment form one person would be received to your account, from where you will be able to transfer the funds to the third party.
Not difficult, but well over your head if you don't even know how to receive funds to your account through PP.