Perhaps the sting was directed towards an ISIS cell - like the one discovered in NYC about the same week? In which case, they would only need the few high powered weapons and explosives that Paddock offered. Since ISIS has been caught in a couple of these stings, they had a plan to turn the tables on the FBI should this arm deal be a set-up?
http://nysepost.com/new-york-is-inspired-terror-plot-uncovered-363647
Here’s the report of a similar style attack plan that was stopped by an insider informant several days after the Las Vegas shooting.