How large of an order volume are you receiving for your game? If hundreds of thousands of people are buying your game you will probably need a much more robust system than you will need for handling tens or hundreds of customers. BTW, what is your game? I might like to check it out or buy it. :-)

Well, Um, ummmmmm, uuuuummmmmmmm (<- Me thinking) If you can program with networking and databases (If you can't, just get some books or try to recruit someone), It'd be a fairly simple(But Still Hard) Database that holds all the codes, random generator that created a random code, checks it against the database to see if it's already used, and then adds it to the database when it makes a code that hasn't been used. Then when someone puts in a code you'd have your game automatically connect to the server(That holds the database) and check if the code has been used (If they can, say, install on more than one computer just check if it's been used a certain amount of times). I would compare this to a linkedlist? I'm sure if you can't do any networking that might have been kind of cryptic, but I'm sure any network/database programmer you recruit would be able to do it.