It's not too difficult to imagine that users of particular sites would forget information vital to them buying goods and such. I personally forget things quite often. Recently, I had received a request from a site user that they had forgotten not only their password, but their user name as well. Making password recovery, and ultimately product purchasing, a bit more difficult. If a lack of proper support is given, then bam, you've just lost a potential buyer.

Go off of the information that you're given. With this instance for someone buying from Howard Huge, I had their name and their Email. Still very workable so long as the information is accessible through the database. Passing along the information, a new password was crafted along with a way for the customer to change it as they see fit. One has to be somewhat of a detective if you want to get anywhere.