Fishing for a client list. To be fair, asking the store owner "Did you sell a knife to this murderer?" might yield a saner response than asking the killer. Unfortunately NSO are not that helpful shopkeeper.

In this case the killer is also the investigator.

"So I asked the store owner if he sold me the knife (wink), he didn't hear my question (wink), and so I guess I have no choice but to close this investigation".

I also want to point out how odd it is that no one cares about international weapons exports, except when talking about cyber weapons. USA is the #1 arms exporter in the world, much of it goes to tyrants (Saudi Arabia is the #1 importer). Fighter jets, missiles, even nukes are being sold all over the world, but god forbids someone sells an 0day.