It's like having a yard sales and trying to sell to your neighbors that already know what you have to offer. We have to think outside the box and figure out the best way to advertise to people outside of social media groups. Some ways to do this is advertise on your vehicle with vinyl lettering or graphics because not only are the same people seeing your advertisements everyday, so are other people seeing it for the first time. I think of it as opening the phone book and showing the people on the bus your Ad everyday. Some will have already seen it however there will be new people on that bus whom never have seen it.
An other way is to advertise on sites like craigslist.org or upillar.com.
Do a search on "Classified Ads" or "Classifieds" and you will see a lot of sites that let you post an ad for free. Usually the sites at the top are the top sites to list on because they've been the longest running or are most popular. (be sure to never leave any personal information on classified sites) Just leave a link to your selling site. Good Luck!