I'd imagine it doesn't really matter if the seller know's it's going to be used illegally after they sell it, as they've sold it, and really it's the drivers/riders responsibility to have the correct documentation. But I'd also imagine it's up to the sellers discretion to refuse sale, if say case in point, someone goes in and asks for a hornet restricted, when they can only technically ride a 125, he should just say that's the law, not be a bit of a helmet about the situation.
I bought my bike without having a CBT, got the CBT and picked up the bike the day after, but theoretically I could have just bought the bike on the Saturday, turned up and rode off on the Tuesday, all the shop needed was my cover note to tax it for me, they didn't need my licence or my CBT certificate.