I grew up in a "part time" gun shop that outsold the local full time sales shop. News paper advertising was a crucial aspect in establishing, however that was a pre-digital age time. It did take time to build a customer base but it didn't take long by paying a fair price for used guns and then selling them fairly at a relatively modest mark up, especially for "hunting class" rifles that were in excellent shooting condition but might have had some honest use cosmetic flaws. Occasionally a unique, rare, or collectible would walk in and present an opportunity to sell nationally at a little more mark up. Customer was happy as he got an honest local price and often lacked the ability and knowledge to get into the market that would really maximize value. And we were happy to do the extra leg work for the extra profit. Our key was in an increased volume with a smaller profit margin vs an increased profit margin.
In today's world I would still utilize newsprint, but probably only seasonal and instead focus more heavily on digital media.
The other aspect that contributed greatly to our inventory was a pawn license, but this was unique to our rural area that had a high seasonal income differential. Here is the pattern that was observed over and over. Early fall a customer would come in and purchase a rifle for the upcoming hunting season, hunt all season with it, then either pawn it or sell it back in early to mid December (money for Christmas)(if pawned we saw less than 50% return for their gun) and then come back in the next fall and start the process over. We didn't make a lot on each gun but made some, more importantly we got word of mouth advertising.