As the seller you are required to declare to a prospective buyer that the car is Category S. If the documents you supplied with the vehicle indicate that it is Category S, then it could be argued you have done so, although this would depend on legal interpretation if it went to court.

However, if there was nothing in the paperwork to indicate it was Category S, the advert used to sell the vehicle did not indicate Category S and you did not say directly to the buyer that it was Category S, then it would be possible that a court would decide you have withheld this information.