Which payment provider will allow payments directly between two third party customers with no intervention from me?

I need a payment provider which will allow me to coordinate a fee paid between two users of my website, but I would prefer not to have any part in this (makes legal/compliance/tax issues easier). Is there a provider which would facilitate direct payment between two users, take any fee which it stipulated transparently, and leave me out of the loop? Paypal seems to do this (e.g. with ebay) but I can't find any reference to how it might be integrated into a site this way on their developer pages.