How to assign unique account numbers OR usernames?

[FourX]

What purpose does this serve?

I sell digital art downloads for others to use so long as they have a valid license. I need to be able to issue a unique license to each customer, it lets me and others know that they purchased the art ligitimately and they didn't pirate my art. So I really NEED to be able to assign each of my customers a license number, it can be short, 5 digits preferably, and if you can help me or direct me to a mod that can give each customer a unique id number that would be very helpful