I'm sure gun safes in the apocalypse would be of utmost importance. Isn't it limited to selling only 3? That is the way most things are. My point is, it takes an incredible amount of resources. You are acting like its an easy exploit like crafting a picnic table was, I rarely have 40 forged steel sitting around that take ages to forge and mine the metal. You earned it. You can make a living crafting and selling stuff, I don't see the problem. I sell my tier 3 axes when I craft tier 5's, etc. I've crafted spares and sold them before too.
Arguably I think I could make money faster looting and selling by far, so why is this an issue? To me if you cal sell more than 3 at once I'd want to fix that so you'd have to travel to several traders to unload all your safes, but if you want to role play as a safe builder because its good money... more power to that guy, as long as it isn't the best way to make money. The best way should be the most dangerous way.