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.