Mzybe he just really liked the firearms scene. My guess on this is someone he thought was a friend wasn’t a friend and ratted him in exchange for something. That’s usually how this goes.
The more sinister idea, and have pictures of him at gun show tables int he reports over time, is he got caught up in a general net that’s just looking for people to bust.
Some of the quotes from the report say they didn’t know who he was at early on, but they had suspicions. That’s happens more than people want to believe. Moles, informants, agents working shows noticed he was standing out and decided to make an eample of him.
I'm not justifying the raid, or his being shot, but the laws are what they are. If you buy multiples of the same make/model of firearms, sell many of them within days of transfer, that is what ATF looks for. If one or more of them shows up at a crime scene, or in the hands of prohibited persons, you're making it easy for ATF.