I’ve never used apps for this purpose. I’ve sold books on Amazon and held garage sales. I know a lot of people who have sold things on Craigslist.
You can do a single blanket posting on Craigslist, listing the things you have for sale. Post lots pictures, or you won’t get any interest. It’s all very tedious, but the more work you put in, the more likely you are to have sales, unfortunately. I know a couple of people who have sold a lot of stuff on Ebay, but they did single listings for everything – lots of maintenance.
I’ve also seen people sell entire contents of their apartments via university mailing lists. If there’s a big university in your town, see if they have an active message board / forum / Facebook page / whatever. Newly arrived students and postdocs need cheap stuff that they can eventually sell on down the line when they finish.
A garage sale probably means the least amount of prep, and you don’t have to wonder whether people will show up to buy what they promised to buy. That’s the biggest drawback of Craigslist. But with a garage sale, you have to hang out and watch the browsers. And if you’re not on a well-beaten path, you have to advertise.
There’s going to be a trade-off no matter which option you choose. All in all, a listing on Craigslist or uni site is probably the easiest route for the biggest return.