I have zero experience in the retail gun business, just a perspective as a customer.
IT would seem one way to get inventory cheap and move it is the used gun market, especially estate sales. Maybe you can't afford to buy 100 new guns from a distributior to stock your shelves. But what about buying a lot of 20-50 from and estate sale, or a police sale? Maybe clean them up a bit and sell them on the internet and storefront. USe some of the money to buy a few new guns.
Think about who buys lot sof guns and paraphenelia. IT seems that shotgunners run through a ton of shells and clays (which they buy at Wal-Mart). But they also have vests, wear out guns, etc. Or self defense. Glock is a no brainer. Or be the shop that carries the best selection of new and used 1911s (lots of gunsmith opportunities there). One shop in my area carries only Browning, but has EVERYTHING Browning: shotguns, rifles, pistols, all the hunting clothing, flashlights, everything BROWNING and nothing but BROWNING. Another specializes in black powder. Another in reloading equipment and supplies. Another in a crappy attitude.
Another specializes in police sales with all of the leather belts, cuff cases, cuffs, flashlights. Anything and everything for the cop or security guard, plus some of it is even of interest and use by civilians. I wish there was an M1 Garand specialist nearby...Hmmmm...what if..
I would find that niche and then try to find a way to get into it with used inventory. I think services and accessories would be more profit with less upfront invesment than firearms, but you need some firearm focus to make it real and a focal point for coming back. As a reloader I know we go through a TON of supplies, especially bullets which are kind of expensive to buy by mail.. I buy all mine at shows or local shops. Lot of gun powder. It really helps if you have someone on staff that is knowlegeable about reloaindg equipment, processes and supplies. Carry the books, bullets, powder, cases, and equipment. Are you supplier the benchrest guys with precision single stage presses, precision rifle components, and electronic scales? Or the comabt pistol competitor who needs a Dillon progressive press, lots of attachments, bulk pistol components, and more emphasis on the leather gear to compete with? Or how about the cowboy action shooting market?
There are several shops with no focus whatsoever and they can only compete on price. But if I found a shop that was knowledgeable and helpful with one of my pet interests I would spend most of my money there even if they did not have the cheapest price.
Stay away from carrying as your main product what can be had at the local Wal-Mart or discount shop. You can NEVER compete with Wal-Mart on price. So carry what they never will. Everyone carries Speer bullets, so carry A-Square, Barnes, Garret, Buffalo, etc. Don't carry Reington 710. Carry the Super Grades, or the Varmit Masters, or the tricked out AR. One guy on the board here was trying to find a NM AR15. All the shops were only stocking M4s. If you specialize in NM gear then go all the way and expect to market to a wider geographic area and get an online shop as well.
Best of luck to you. I think the others are right that if you go about it the normal way you need $750,000 not $75,000. But, I also think that if you are very focused and creative in using old inventory, quality accessories and supplies, and can find a very cheap storefront (or just start with the internet) then you could be successful. If you don;t have specialized expertise then start getting it. Become a Glock armorer. Take some machinery or gunsmithing courses. Build an AR from scratch, Sporterize and Enfield. Start reloading. Try out SASS. Something to build experience and eventually expertise.