I am fairly sure that this is Castaway, a 1934 novella by James Gould Cozzens. The protagonist, Mr Lecky, is alone in a huge department store. It is never explained what has happened or how he came to be locked in the store.
The guy was mortally afraid of someone or something that was chasing him
Lecky is nervous from the start, and becomes terrified when he discovers that there is another human roaming the department store -- and still more so when he realizes that the other man ‘could hardly be altogether sane’.
spent a lot of time getting guns/ammo from the shopping complex and building a fort.
In the first chapter, Lecky equips himself with a shotgun and shells from the sporting goods department, and in the third chapter (entitled ‘The Fortification of the Lavatory’) he improvises a fort around a toilet.
He couldn't figure out what caliber of bullets to use for the gun he found... and had to read the store chart to figure it out.
Several pages are devoted to Lecky's attempts to select and load a gun -- a frustrating business, since he knows nothing about firearms. Eventually,
Mr. Lecky noticed on the far counter a heap of small booklets. He let his axe go and, taking one in his damp, unsteady hands, read INSTRUCTIONS FOR PURCHASERS on the cover.
With the aid of the booklet he manages to load a shotgun.
Anyway, the publishing date was probably pre-1980
First published 1934; I had a hardback edition published in 1968, so it evidently remained in print for a while.
The book itself was a soft-cover, maybe 200-300 pages.
My hardback was around 120 pages, so 200 might be about right for a paperback with smaller pages.
The world seemed to be today's world, just without any other people being mentioned in the story.
The only characters in Castaway are Lecky and his unnamed adversary. The setting seems to be a normal, large 1930s department store with no anachronistic features.
The store had stuff like canned peaches, and seemed to be in immaculate condition.
I'm not sure if canned peaches are specifically mentioned, but Lecky definitely lives on the non-perishables available in the food department, including cans and glass jars of fruit.
The ending was that he killed the thing that was chasing him, which appeared to be his clone/himself.
The ending is rather surreal: Lecky succeeds in killing the other man (mostly referred to as ‘the idiot’ in the text) by shooting him with a shotgun and cutting his throat with a kitchen knife. At the end of the book, the dead idiot is somehow reanimated and crawls up the stairs to meet Lecky. It is not explicitly stated, but very strongly hinted, that Mr Lecky and the idiot are in some way the same person.
The book was notable for never using the main character's name, just calling him The Man and never used first person.
Certainly the book never uses the first person, but the main character is referred to throughout by name as ‘Mr. Lecky’ (at least in my copy).
The cover had a picture of a scared-looking guy holding a revolver on it, while running.
Here is the cover of a Bantam paperback edition.
(Not very faithful to the text, since Lecky doesn't wield a revolver at any point as far as I can recall.)