In order to find out the buyer's e-mail address log into your PayPal account, locate the transaction in question and click on the "V" icon beneath the amount that you were paid. This will expand the page and show you the full details, including the buyer's e-mail address.
Once you have located the e-mail address click on the "Send And Request" option at the top of the screen. This should then bring up another line of options. Click on the second one from the left, which is the one that says "Request." Type the e-mail address of the person you want to request the extra payment from, then click on the button below that line that says "Next."
Unfortunately given that I no longer sell on eBay and don't have anybody that owes me money that I can send a PayPal request for money to, I was unable to get any further than that when I tried it from my own PayPal account, but if you use this information to start you off then hopefully the remainder of the process will become apparent as you progress through each step of the "Request Money" process. Failing that, perhaps another responder will be able to give you a more definitive answer to your query.
