It sounds like the prosecution is not for selling an ebike /moped but that the bike wasn't supplied with a certificate of conformity documentation to allow the seamless path for approval to become a legal roadworthy moped. The conformity means it must pass certain criteria /testing without the need to further modify it.
Under the current guise of selling it, there is no frame work to follow to allow it's legal use, all S -pedelecs from leading brands will all supply a cert of conformity to show it meets all EU legal requirements.
One suspect all these 1kw hub bikes sold also have no cert of conformity either but whether those vendors will be collared as well, who knows.
The cyclotricity bikes also should fall under the same DVS scrutiny as they are illegal with no way cert of conformity.
Vehicles that need registration as a motor vehicle if they are to be used on the public roads, can be legally sold without a CoC or even a VIN number. The supplier can only be prosecuted if they claim they can be used on the road as they are.
The proof of this is that one can apply for a CoC or VIN number for vehicles not having them, using systems set up for that purpose by the DfT. Of course they have to reach the necessary standards to have a CoC.
In the case of a VIN number being needed, the DfT usually needs to be satisfied that the vehicle originally had a VIN number when it had been lost in some way due to conversion, major repairs etc. However this doesn't apply to kit cars or vehicles originally never intended for road use but repurposed by alteration for that use. These are issued with new VIN numbers if someone wants to register them.
For any remaining indeterminate cases, vehicles are issued with Q registration number plates to record the uncertainty.
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