I've used Amazon. One case - Google told me the app did not work on my device. Funny thing, I was trying to update the damn app. It did work. The updated app I installed from Amazon had no problem. Google now sees it as current and wants to update it.
The other 3 apps from the same mfg, using the same software updated with no fuss. I simply re-purchased from Amazon for 99 cents.
Amazon was all right until this last year when they went to the new format.
Appbrain has a better description of some apps than the Play Store. But if you want an app, you usually have to go to the Play Store to get it. There is one app store you can use if you don't have Google. I've seen it in some browsers. I can use midiapolis on my ereader. It's Android, Polish, and it doesn't have Google, nor do I want Google.
I buy books from independent sellers only.