Let's say that I wanted to start buying or selling on SR/BMR/SMP for the first time ever. I start account on SR/BMR/SMP using Tor. Is that unsafe? Should I start fresh with new accounts after install/using Tails? Or can I use the account I made already using just Tor?
There is no definitive answer on how secure/paranoid you can be.
Did you make your account using Tor on a Windows machine?
If so then 'whoever' could have used some other backdoor to your PC. Is it likely for this to have happened - probably not.
Ideally you would use a clean pc, clean internet, tails, tor, etc...