If you know the eBay account name you can use this link to get user information. By the way it also sends information to the other person saying who was asking for information.

http://cgi3.ebay.com/aw-cgi/eBayISAPI.dll?MemberSearchShow

If someone has a PO box, it must be registered at the PO with a real address. If your friend mailed money and got nothing, I'd head for the postmaster at their location and start an investigation for possible mail fraud.