I had a few other stores before this that didn't sell well at all, and I have to say that's because I just didn't know the products, or what the end users really wanted/needed/cared about.
Great ways to pick products: - Terapeak (http://www.terapeak.com/), but this is paid - eBay completed listings - Or most simple (and what I use) -- once you know your products, search AliExpress and sort by "best-selling". That's my go-to.
Feel free to check my store for ideas (or if you want to buy something!). URL is: http://modshop.guitars/