The game shop a few miles from my home advertises heavily on Offer Up, Craigslist, and Let Go. In addition, when I see something of his on Let Go or Offer Up, I notice he also mentions that he purchases classic games and systems. I rarely go to the store, but about 50% of the times that I have gone, people have been selling him items.
I know he also advertises on Facebook, because people have mentioned, "I'm so and so, and sent you a message on Facebook." I'm not sure how thay works, because I've never used facebook.
So you might have to use apps and Facebook to market the business.
I also know he pays about 1/3 - 1/2 of an items value, and gives 10% extra if you accept store credit instead of cash. People seem content to get that amount, because they probably haven't touched these games in years.
Finally, what I feel helps the most is that he rents probably 1/3 to 1/4 of the space inside of a comic book store. So I imagine his rent is very cheap.
The thing I find most interesting about this arrangement, is that when I'm there, at least 75% of the customers are there for the videogames as opposed to the comics.
Maybe you can share a location with a business, in order to save a bit on overhead?
Goodluck