Can't help you on how to get it home; talk to your DMV about how to obtain a temporary tag or similar.
Seller retaining plates and registration sticker makes sense if his state requires turning them in to cancel the registration or to transfer the plates to a replacement vehicle. That's how it works in Georgia, albeit our registration sticker is affixed to the license plate rather than the window. When I've sold cars or motorcycles, I retained the plate and either turned it in to the DMV or transferred it to my new vehicle.
Plus - if I sell a car and leave the plates on it, guess who gets any parking or red light tickets...