I've never sold a rifle and included a magazine. I've never bought a used rifle that included a magazine.
Reading that add, it would never occur to me that a magazine would be included... I would have definitely expected to get it without one. There is not one in the pictures, and it says nothing about one in the text.
It is a used rifle. It is not a new one from a dealer. There is no reason to expect that the "box, lock, mag, manual", etc, that come with the rifle from the factory will be included, unless specifically stated. Everyone has their own preference for rifle magazines.
That add is SOP, and the seller is in no way out of line by not including something that was not advertized. The seller can't forsee every possible buyer's every possible assumption. He placed what was for sale in the description and pictures, and delivered exactly that. That is what you want in a seller. As a buyer, if you are wondering about something that is not in the text or pictures, it is on you to clarify. As a buyer, I always clarify even if something *is* in the pictures, but not the text. As in, "hey buddy, that is a good price.... is everything in the picture included? Even those Troy sights?"
You want more than one crappy 5rd factory mag anyway.
If I'm buying a new rifle format for the first time, I go to the best source and order 10 mags, often before I ever receive the rifle.
I suspect this is your first internet used gun transaction? Next time your process as a buyer will be refined. No worries.