change short name?

simple - create a new user account, with all the correct information you want there, copy over your music and photos and such, get the new user account all set up exactly how you want it, then delete the other account.

the most prudent thing to do would be to wipe the hard drive and reinstall the system. who knows what exactly was done to the computer while it was on display. I wouldn't take any risks.
If you don't want to do that, as was already suggested, create a new account with the name you want and migrate your data.

the most important things to move are mentioned in the this [post|http://discussions.apple.com/message.jspa?messageID=6185507].

The best way is to go to the user preferences. ctrl-Click the account that you want to change and click advanced. change the shortname and home folder path. The system will create a folder with the new shortname as your homefolder. Transfer EVERYTHING from the folder with the name of the old shortname to the folder with the name of the new shortname. Thats all.

If he's not going to use the old account, why rename it? Best ideas (as suggested) is to Erase and Install before using the machine (if the install disks were included with the purchase, as they should have been), second best idea is to create a new account and delete the old account. Using the advanced preference is useful if the user has spent a lot of time setting up the machine and now wants to change the short name.