Col,
There is no legal bar to selling a dongled ebike, only legal restrictions on its use.
As I said, it could be bought to use as an exhibit in a museum of modern art.
In that instance, the item is not being sold as 'an ebike to ride', but simply as an item to exhibit in a gallery.
Such an exhibit does not need CE certificates or any other certificates, so all that is irrelevant to the sale transaction.
There are some things which are illegal to possess, which will also almost always make their sale illegal as well.
It is illegal to possess cocaine, and it is also an offence to supply it.
It is not illegal to simply possess a dongled ebike, and it is not illegal to sell one.
You can keep hammering on about dongled ebikes, but 'illegal sale' is not your best point.