Ninstints,
If the XP license you installed was a retail license purchased in a pretty retail box, you are within your usage rights to move that license from one computer to another.
If the XP license you installed was an OEM license, then the End User Licensing Agreement prohibits moving the license to any other computer.
From a more immediate perspective, even if you have a retail license, you made a big mistake if you did not format the disk after the buyer saw that the computer worked. There would be nothing to keep the buyer from activating the computer once they got it home, and there are readily available utilities that they can use to retrieve the Product Key. If the buyer activated, when it comes time for you to install and activate XP on your computer, your activation attempt will be rejected and you'll have to tap dance on a telephone activation.
