Can I ask, did you trade your old car in with a car dealer?
A trick that "some" dealers do to retain cash flow is they accept your car as a trade in but do not clear the finance until a perspective buyer purchases the car. For a dealer to sell the vehicle they need full title of the car, but many do this as a way of keeping their money until the last minute.
If this is the case speak to your old finance company and explain that the vehicle has been sold. I am not positive on this bit but i was sure it was your responsibility to clear any finance owed. A lot of dealers offer to do this on your behalf building in the finance repayment with the sale.
A dealer will always do a HPI check on any car received so they will know if there was finance owed.