You can generally assign an asset tag in the BIOS, depending on versions. I can't give a specific answer without knowing the BIOS type. If you're selling Dell or I believe Lenovo systems, you can assign tags quite easily.
This article details how to use Windows-Shell-Setup to enter various information and details about the system seller in the Windows Environment itself. https://technet.microsoft.com/en-us/library/ff715801.aspx