do a google search on just the first part of the email address without the email extension i.e. "user name" @hotmail.com. Many times the person will use the same "user name" for multiple sites. I had an offer come in via a parked page, with a little research (the tactic above) I was able to track down all this info on him: age, city he lived in, marital status, number of children, hobbies, where he worked etc. (all free via google searches) Never ended up in a sale but I knew everything (nearly) about the potential buyer which goes a long way in being able to negotiate a good deal.
best of luck