You probably won't believe it, but, yeah! I've seen guys behind counters in gunstores, and vendors at gunshow who have no earthly idea about military calibers. I've seen them try to sell 7mm Remigton magnum to guys buying a 7x57 rifle. I've seen folks try to sell 7.65x53 mauser as 7.62x54R. I've seen vendors argue with people, telling them the 7.62x39 is the "same thing" as 7.62x54R.
Caveat Emptor.
Learn as much as you can, dealing with firearms can be dangerous for the uninformed.