Most collectors have a decent amount of ammo around the articles and picture shows no ammo. And these look mostly like modern massed produced firearms rather than collectables. My bet is that he was an illegal seller of firearms - a gun runner.
If this were a "gun runner" you would see far more handguns than is pictured and they would not be of the type in this collection. Criminals are not going to be interested in a bolt action No4 Mk1 British Enfield rifle.
This guy is a gun collector and this is a collection many collectors would aspire to own.