I don't know if there is grace period to return a purchased car, particularly simply on the basis of the buyer just changing their mind. It would seem that if you could do that, people would be buying cars left and right on Friday nights and returning them on Monrdays for free?? You would have to speak to an attorney about whether there are any legal options. You can find more information about an attorney through the attorney locator service on LawInf's home page at:
http://www.lawinfo.com/. Some do offer free consultations. Good luck.