you’re making an assumption that all the guns in the room were purchased by him and all the guns purchased retail were used. The latter is not true - none of the guns he purchased from one shop were used according to that shop; the former has yet to be disclosed to us peons.
Exactly. All we really know is that SCP purchased some guns which were not used to massacre the crowd.
Determining who purchased the ammo and brought it to the room will define SCP’s level of participation.