I tried this. I bought a phone with cash, bought a prepaid SIM with cash and a google play card with cash.

Used a throwaway email address, and I could not get the play credit into the account.

I even opened a support ticket with google and they could not help.

I am an IT professional and quite adept at navigating technical bureaucracy and wasn’t able to do it completely anonymously.

My guess (and it’s just a guess, i could be very wrong) is there is some internal, undocumented check that the account has been tied to a real person before it allows credit onto the play store.

If someone has had success doing this I’d be quite interested in hearing about their experience.