When you sell a gun in a face to face encounter, ask to see a Driver's License. You can tell the buyer you need that for your records. It will also show whether the buyer is local or not.
There are state laws against selling to people from out of state.
If there is ever a trace, you have the information to lead to the buyer. He may not be at fault, he may have sold the firearm to someone who used it in a crime. Your lead may help find the bad guy.
If you are buying/selling/trading guns on an active basis for income, you not a collector, you are a dealer. Those patterns will be obvious to the BATF regardless of what you do. The paper trail will come to you.