Does buying thousands of apps (but not installing them) cause bloat in Android?
No, they are not installed on the device... Your device doesn't even "know" you purchased the app. It has absolutely no effect on your device at all and your purchase history is stored on Google servers, not your device itself. 

Your device has nothing to do with the "purchasing" of the application if you don't actually install it. You don't even need a phone to buy a free app, you can just go on any computer, log in with your Google credentials, and go to the Play Store and have the free app added to your Google account.