It's the buyers responsibility to know what they are doing. NOT the vendor. They don't need to or (last I checked) are bound by any firm to inform the buyer of what he/she should already know. These aren't toys and if you have some "noob" friends getting into the AR hobby you need to be ripping THEM a new one and encouraging THEM to educate themselves on the plethora of laws pertaining to whatever weapon systems they are interested in. The implication that dealers need to babysit buyers is ridiculous, I thought we were all adults.
As far as refusing the charge, that's nothing more than fraud. If you bought and recieved a product and feel that secondary to the dealers refusing to coddle the buyers and have regular penal code study sessions then return for refund. Refusing the charge and keeping the item just shows an attempt to justify nothing more than weak moral turpitude.
Slayer of abalone and lingcod.