Appropriate due diligence isn't perfect, but it is prudent.
It would be silly for me to keep arguing the point, but...
Let's say you saw a gun advertised on this site.
By a member of long and good standing.
It is a model you have coveted for some time and the price is reasonable.
He/she lives near your town.
You have numerous conversations via PM, email, and telephone; regarding the options for payment and delivery.
It is decided that you will meet at an his/her office.
You are greeted by a person that identifies themselves, verbally, as the person you have been conversing with.
You sit in their office discussing gun stuff and chit-chatting.
You notice the diplomas and certificates on the walls match, by name apparently, the person to whom you are speaking.
The person's secretary or nurse buzzes in to say that there is a call waiting or a patient that needs attention.
You are asked to wait in the lobby/waiting room for a few minutes, while business is conducted.
After a short wait, you are invited back into the office and complete the transaction.
The seller does not offer a receipt, and when asked, states, smiling, that "I'm one of those whacko militia guys that doesn't do the paper-trail thing".
You leave w/o the gun?