I believe it's mostly a state-by-state thing. The state laws I'm familiar with apply to "precious metal dealers", defined as people who are "in the business" of buying and selling. The laws require a state license and cover things like customer KYC, mandatory transaction records, criminal background checks, and restrictions on transactions with minors.

The critical distinction is between someone who is buying and selling precious metals, compared to someone who is "in the business of" buying and selling precious metals.