There’s nothing illegal in selling or owning one, and you can use them on your own land or ride around dirtbike tracks as much as you want. But I can guarantee that nowhere will any of the sellers be found claiming using one on public roads would be legal, because it isn’t.

This is BTW the exact same reason why you can buy electric scooters in the UK even though they are illegal to ride on public roads, as that is the only illegal part.