Hi, I think it could be a Unitex Chamelion which were previously known as seejen, if you look on the gallery section at Born Free you will see the same lines along the hull, i can not remember a jet drive boat being built though !
Also the advert says 21ft the chamelions were only 18ft 6in but i could be wrong, the seller says its a Fonda but i have never seen a Fonda with those lines.