You do realize that Windows 7 Professional, Enterprise, Ultimate all sort come with a copy of XP. The only reason I say sorta is because it doesnt actually ship with them you have to download them free from MS. But i dont understand what you want Best Buy to do for you. They can only sell what the PC manufactures give them. If they dont give Best Buy a XP option Best Buy cant sell a XP option. But keep this in mind the Windows 7 to XP downgrade is available through major OEMs for a limited time on qualifying PC's. The qualifying PC's are usually ones that already have Windows XP drivers available. so if a model has XP drivers available but ships with 7 you can request through the manufacture a 7 to XP downgrade.
----------ALWAYS check your purchased items for damage before leaving the store. If you never leave the store and check it in front of a employee it removes all doubt that you might have damaged the item.----------