Local gun guy/friend has been doing it around here for quite a few years. Told me lately that the biggest problem he has is that many folks don't subscribe to a small local paper anymore. He now lists on local online Buy and Sells too. He doesn't run his adds continuously, but often, and tries to run them when the local advertiser papers list the local upcoming auctions or when folks need cash, like before/after Christmas and tax time.
Like when buying anything and reselling for profit, one needs to know real values and what their customer base wants and is willing to pay. Buying neglected/broken firearms may be fine of you are a gunsmith, but depending on the firearm, may not be worth it otherwise, unless you have a buyer for parts. Another consideration is competition. My friend says the money is not in it like there used to be, between online used gun sellers and the competitive prices one now finds on new guns. Market has changed too, with many of the older, cheaper guns not really desired anymore. Used to be a time when dad's bought their son a cheapo MilSurp for their first deer rifle. Nowadays one can get a new namebrand that shoots 1MOA outta the box for less than $400 and dad's see it as in investment for the kids lifetime.
Still, you have the license, the cash and it seems the desire. Only practical way to increase your inventory is to advertise. Only one way to find out.