The young bucks here all ride 600's and up, usually in collared shirts and jeans. Its an education issue, absence of NGO's and Public Service type PR. That being said I jumped on a 600 soon after qualifying in the UK, and over six months didn't have a drop, didn't have a near miss, didn't have any drama. Of course it was a Honda, so it was pretty much riding itself

I don't know if a bike salesman should try to educate a buyer on bike size, or turn down a legal buyer with cash in hand. So who failed here? I don't think its the dealer. I've sold things to people for yachts that don't make sense, so there's no point in arguing about it. "I'm not sure that's a good idea" and leave it at that. It's a legal sale, and your reputation could just be as easily damaged for not making the sale as selling a dangerous (but legal) product.
Maybe you should have to bring your new bike to an Advanced Rider session. A police rider shadows you, and checks your roadcraft, and says "be careful son, that's a lot of bike considering how new you are" and then lets you on your way. No smiling. No smoke-blowing. Only a bit a truth. It's too much bike son.