I buy old gun books for my library. I prefer the older writings, so I think you will continue to do well. But I would suggest that books and magazines are likely secondary purchases. Don't be disappointed if many folks want to look for a certain arm or ammo before checking books.
Other suggestions:
Usually price is based on cost to the seller (you) and knowledge of desire (a book by Elmer Keith would likely be more in demand than a book by me). As in all books, 1st Editions tend to bring a premium, as do autographs by the author. If you cannot remember your cost, (I can't either) you just have to wing it.
At gun shows, everyone seems to want to haggle - including most vendors. So have the marked price more than you expect. I tried marking some items with what I expected and found I was constantly offered less. And vendors would almost always take less than the asking price. It's the game.
Decide beforehand about prices. I put hang tags on items to show asking prices. On books or magazines, this may not be desired. (I would be disdainful of a Shakespeare early folio with a gummy sticker on one corner. Nor do I want to rip half the cover off an August 1923 edition of American Rifleman while removing a price sticker.) Have two lists; one to allow buyers to see what you want for each book, gun or doo-dad. Have a separate list (not to be seen by customers) of both what you want and for what you will take. It will speed up your thought process and transaction times.
Find a nearby show and meet the promoter. Our gun club sponsors the local show, but there is a member of our club who is the permanent 'ram rod' of our shows. Book as far in advance as you can; you'll get better table choice. Typically you don't want to be next to more book dealers. Or whatever you focus on selling. At first at least (maybe always) pay up front for your table or tables. It just gives you a better feeling.
Bring your own folding chair. Normally the venue will supply chairs. The metal folding sort that pinches fingers and gets hard by day's end. Have a blanket or table cloth to cover the table before you put your wares out for display. It really gives the appearance of being prepared not a fly-by-night. You might consider bringing your own coffee and cup. My club gun show offers free coffee to vendors but it comes in a styrofoam cup. If one requires a special diet (Kosher, salt free, vegetarian or vegan, halal, diabetic, etc) fix your own lunch prior to starting the day. If one knows one is going to be gone for any reason, let the promoter know prior to departing. (Heart attacks don't require advance notice.)
Above all, be polite, friendly and honest. Stand up people and scalpers both get identified and known very quickly.
