Spotted...

Well frankly, it would be a good idea to teach him a little bit about how to ride a fixed gear bicycle before selling it to him, even just to tell him that it's usually a good idea to have strap and cage.

It's not Bellas job to teach the guy how to ride his bike...if he chooses to buy a certain kind of bike without finding out anything about them first it's his problem.
When i sold my house i didn't tell the woman to make sure she locks the doors and windows when she goes out.