Power
Besides the hand cannons and arm rockets adorning your shelves (or sitting in the safe underneath your bed), you're armed with the U.S. Constitution's Second Amendment―the right to bear arms. And no, not the furry kind.
The best thing you can do as a gun dealer is to know the laws and follow them. This includes knowing when those laws change and adapting to those changes. Take our word for it—this will protect you more than even the most high-powered of rifles. Just because you're free to sell a dozen extended magazines with a nod and a handshake on one day doesn't mean it'll still be legal the following Tuesday.
Power is sort of the misidentified elephant in the room when it comes to gun rights and laws. It feels like a powerful thing to own a firearm, especially when it comes to the purpose of the Second Amendment—a state's collective right to self-defense against the federal government or other intrusive institution.
But the power really lies in the laws regulating firearms, and you'll have no direct control over those as a store owner. Such is the case with representative government, like it or lump it.
Most gun control laws are created or removed at the state level, and court challenges to those laws can reverse or re-reverse laws quickly. If you're getting into this business, you'll want to study up.