Yep, can be. Its a Messerschmitt kit car by Tri Tech,'CONSTRUCTED IN YEAR x TO THE STANDARD LATER PRODUCTION RUN, SEE LEAFLET ATTACHED' if it were me. 'Please note that this car has been based on a 1959 Messerschmitt registration but is modified to Tri Tech specification'. If the insurer has issues they are going to come back on that to ask. It will be insured as a Tri Tech. That is legal. The insurers are not responsible for tax grouping.
You can argue that a Tri Tech does not gain enough points to loose its Q plate to a dated Messerschmitt plate. The fact is quite a few Tri Tech are on Messerschmitt logbooks. It is between you and the insurer to get legal to their satisfaction and yours.
Where it becomes illegal is to dodge the MOT. The car is not pre 1960. No MOT, no insurance cover. That might stimulate the Police to investigate the registration if pulled, and if they feel like i, present a case for fraud to avoid taxation.
Of course buying from eBay and getting done in the next few weeks means the buyer can bounce the charge back on miss selling. So the seller gets done for fraud, or deception, as well.
THe cunning could 'buy' this car with a withheld card payment or somesuch on a cash deposit and then place a deception claim into court against paying the balance while holding the car. A game of bluff, coming in, there. Is the penalty worth the fight. Be a cheap car then.