no, but you need to pay for the time that the registration ran out to when the change of ownership was carried out, but if its a few days the post shop dosent worry about it (personally happend with me)
the sales yard has to provide you with a car that has atleast 3 months rego and a WOF that was issued recently before you get it (i think its 1-2 monts).
short answer, hes having you on so if you tell him that you cant be bothered with going through with the hassle of the sale and want to pull out im sure that he will start singing a diffrent tune