In every State, gun-dealers(Licensed) must sell the same way they do in their place of business. Being at a gunshow does not exempt them from the Federal Firearms Laws or any applicable State laws. Out of state Dealers at a gunshow are under the same laws as the state they're selling in.

sales between private individuals- All applicable state laws must be obeyed. For exampe, here in Ohio The buyer must present a valid Ohio Driver's License with his/her photo on it. further, you, as the seller, must write the information on the License down. although this is seldom done here.