Umm, you do understand, that statement goes against what you're selling?
People who have no business carrying (as you put it) would not understand what an appropriate response would be. If people carry and "don't mess up", it sounds like they're doing just fine.
Look, you're welcome to seek out more training if you want it. God knows I see guys advertising courses. If Joe Shmo owns a gun and doesn't misuse it, that's completely fine. He doesn't have to take a state-sponsored "welcome to firearm ownership" class, nor does he need to enroll in Captain Randy's Tactical Response For Operators course.
If I buy a gun, I'm paying the seller for it. I'm not giving you (or any other 3rd party) money, too. And if I want to learn something about it, I will do the proper research myself, whether it be cleaning tips, effective use and limitations, proper storage and maintenance, etc. If you don't like that, well,... tough. It's not your decision what others get to do. And I say that without malice or hostility, but I do say it quite firmly.
And let's be clear about something- "required training" would be a mess. It would have to be either watered down to "this is a dangerous device, don't point this end at someone", which is listed on manuals (and often on guns) and useless (other than to take money away from someone); or it would be hopelessly convoluted and complex. Manual of arms on a Glock handgun does nothing to teach a person about an AR, for example. A course on operating a pump shotgun would be useless for a 1911, and both would be irrelevant for a double-action revolver. If you want to "train", you would need specifics for the device involved. Something as simple as "safely clearing the chamber" can specifically be a whole lot of different things. You might say "I don't mean that"... but then, why not? If you're going to train, then that should be step #1. Because if it's mandatory, and NOT covered, you become liable. So what happens then, is you get specific "training courses" (all at a fee, of course), for bolt action rifles, lever action rifles, semiauto, pump... and handguns- revolvers, DA, SA, DA/SA, DAO, striker, hammer, different design features (Sig layout vs frame-mounted, slide mounted safeties; guns without safeties, etc). Hey, I'd like to pick up a Derringer, I guess I need to enroll in the Derringer Training course... what do you mean it's not offered this month? And why do I need a course on DAK triggers simply because there's less controls and a different trigger than a standard P226? Because you need to get extra cash for Christmas shopping?
Just way too much mess to go into, that people have overwhelmingly been fine without.