To them I say: you missed the point. He explains the firearms the way a sommelier would explain and recommend wine, even to a wine connoisseur who likely knows everything there is to know about wine. From the front of the store, it seems he's an actual sommelier as well, and this is the schtick he has adopted for the business he does for the assassin's guild. Wick plays along as if he's ordering wine, saying things like "I'd like a tasting," and "I need something for the end of the night. Something big, bold." Who knows, maybe if you don't play along, he won't sell you anything. This isn't the gun counter at a Cabela's. Also, John may know everything about the gun platforms, but he wouldn't know the exact custom tweaks that have been made to each gun, so those are certainly worth an explanation.