Amazon sells so-called 'legal highs'. You can also buy as many packets of codeine based painkillers on Amazon as you like, delivered to your home. So much for restricting quantities.
Should / could? Amazon be stopped from selling 'illegle high' drugs. For many years I was of the opinion that people should be protected against themselves; but I'm afraid to say that the older I get I think: if that's what you want to do, get on with it. Bit harsh, I know.