Perhaps that's a way forward, treat them like alcohol with sale requiring either evidence that it's for private use or that it's registered and insured beforehand, with the seller being obliged to keep a copy of the evidence.
Yes - exactly
There was a shocking programme on the telly last night about knives - including so-called Zombie knives
they had thousands on show - and most came from the same supplier
One of the mothers - whose child had been killed with one of them - contacted the owner of the company asking him to stop selling them so no other Mother went through when she did
his response was that they were sold to people who just collected them for display
The Police literally had examples - including CCTV - of them being used to kill people
OK - that is more extreme - but the same concept applies
if you sell something under circumstances where any reasonable person knows it will be used illegally
then you should share some of the responsibility for what they do with it