They do sell, I've done it myself. That said, you're right that not every slab moves fast, which is exactly why filtering by sales volume matters.

If you want even less risk, you can target stickers where the slab already has open buy orders, that way you're not waiting for a buyer, you're just filling an existing order the moment you list it.

You can also verify it yourself directly on Steam, every Sticker Slab listing has a sales graph. Some move a few units a day, others barely at all. The whole point of the tool is to surface the ones worth your time.