I can't do this on a new firearm.
A used firearm can be bought in my mind, the used glock 19 would be sold for $400.
I am willing to pay $240 used, where I am in Kentucky is a low income county and the competition to the only 3 gun shops 40-70 miles apart is two pawn shops. Who is beating whose prices?
Will it upset someone, ABSOLUTELY yet I am buying low to sell low.
Thinking people will beat a path to your door to sell you their used Glocks for $240 that you resell for $400 isn't a business plan, its a fantasy.
Based on what you've written, I don't see how you'll break even as a business.
-What is your time worth? Sitting in your shop for eight hours a day waiting on a guy to walk in and sell you his used gun is going to get old by the end of week one.
-Are you going to buy or sell at gun shows?
-You live in a "low income county"? That doesn't bode well for business. I'm in the Texas county with the highest per capita income and guess what.......gun shops fail here every year.
https://en.wikipedia.org/wiki/List_of_Texas_locations_by_per_capita_income
-What is the population within a 10 mile radius? 20 mile radius?
-If you are out in the sticks.... what need do you have for a retail store?
-How will people know you'll give them $240 maximum for their used Glock?
-Do those people know you have competitors that will likely give them more? How do you know what they ill pay for a used Glock?
-How many sellers are going to walk through your doors each day/week/month?
-How much is your inventory insurance premium? Your general liability insurance? Alarm system monitoring?
-Are you going to sell on the internet? GunBroker? Do you have a webstore?
-How many transfers do you expect to do?
-What do you think your business will provide that others in the area do not?
-Seriously, the only guy I would expect to take $240 for his Glock is the meth head who stole it.
I'm not in any way trying to dissuade you from getting your FFL and opening a shop. I'm absolutely wanting you to understand how difficult it will be based on your belief that you can have a successful business based on "buy low, to sell low".