It should not be “easier” to buy a gun in general. All purchasers should be checked for any disqualifying records but that is done by an FBI background check and in some cases a state background check. Some states also require registering the firearm. The background checks and registration are done through a website or phone call. Some states also have unique limits on gun designs. All of this can be done by any gun shop if they want to do the extra work (and I’m sure can charge for it). One of the reasons is imagine - you are a hunter (just go with it) that likes to hunt with 357 revolvers (many do) and you live in California, maybe while flying the scope mount breaks. The only thing you can do is fly back to California, buy a new revolver, then fly back with your new revolver to Tennissee. And the idea it’s hard to know what each states laws are for gun shop owners, they can learn, they can use a website that breaks it down, when registering it can be flagged as “ineligible gun” and the sale not go through. When your job is to sell a gun you can learn. There could even be a federal certification that you prove you learned the rules and only those shops can sell, or sell to particular states.