You know, I'm thinking the vendor glitch might have been intentional. If you think about it, every merchant, even random shopowners who carry no weapons, all carry at least one stash of bullets. Why would they do this if not to enable the "glitch" which could be a hidden feature and not a bug?
I think it's thrown in there to help out construction fans. If you want to focus heavily on settlements and actually building them into walled towns, you're going to go through a shit-ton of resources, far more than you'd ever use to max out all of your weapons and armor throughout the game. Especially steel - aside from Sanctuary there just aren't enough wrecked cars or barrels in each settlement area to build comprehensive walls and defenses. In the absence of the vendor glitch, this would require many, many hours of tedious scavenging.
I have about ten walled towns at this point, and it would have taken me at least a dozen more hours to do it if I couldn't just glitch shipments of steel, circuitry, copper, adhesive, etc from the Diamond City vendors.
I haven't purchased weapons or armor with the glitch though. Those items are 100% my own non-glitching acquisitions.