Alright. Yes the OP did absolutely commit a mistake. he should not have attempted engage in that transaction in the store. However that gun store owner, way in the wrong here. The guy did wait till the shop owner turned the sale down, did not get involved in the sale in any way, so he had that for him. He simply should have followed her out maybe given her his number and told her to call him on the gun and ended it there so there was no transaction on his property.
I am sorry, my LOWEST gun purchase at a store has been $500 and up to near $2,000. If I am going to your store to drop some money, you had best be better than that or I will leave your store and never return. i will see to it my friend never return, I will post negative on your store online for the rest of my life. It is called customer service. Give you customer the benefit of the doubt. Here he had a KNOWN customer who has bought items in the past.
Personally, I do not give a rats backside who other folks have done to him. I am the customer, you give them the benefit of the doubt. he could have easily, calmly and firmly stated, "Please, do NOT do that in my store." Provided you listened to him, then no harm no foul, he has not lost a customer, and he has a teachable moment.
He chose to work in a field where the general public are day to day thing. The general public suck, part of life though. Part of the job is to man up and treat customers fairly, they have no obligation to do that with you and a lot of them won't. I worked at McD's for 4 years and I got more nimrods at my register than I could put in an email. I was never rude (despite getting a couple death threats for cheeseburgers having catchup on them and a filet-o-fish with tartar on it), and I made 4.25 an hour.
There are 2 stores I frequent in my area. This is because the owners are good folk, and treat me with courtesy. When I was a noob, they were VERY patient with a multitude of stupid questions and I mean stupid questions. I commited some fouls (like dry firing a firearm) and they firmly straightened me up on the mistakes quickly. I have spent thousands of dollars with them as a result, and through recommendations probably been responsible for at least $100k in sales (in the last 2 years) from friends and family going there on my recommendation.
IMO, there is simply no cause for pi** poor customer service like that, none. I would never return to that store. I would make sure as many folks as possible knew why.