When I use Northwest check in from home, I don't have to present my ID to anybody. If I use self check in at the airport and go to the check in counter to show my ID, they will just circle the word ID on the boarding pass or sometimes write the word Q/ID (or something like that on it). If you do this by yourself, they won't ask you for an ID. Isn't it easy to use a ticket purchased for another person? You just have to have a his/her credit card or can print the boarding pass from home.