Personally I will only ever buy new brass in sealed packets / packets, or use brass from ammunition that I have bought. I would perhaps use brass from a close friend where I knew it was once fired.
I would never use second hand “once fired” brass bought from somebody I don’t know well as you have really very little clue if it is once fired or has been through several firings, the case has stretched and its thin at the head and will let go on the next firing. I may be overly cautious but I don’t think it is worth taking the risk for the sake of a few quid.
It is however well worth saving your brass till you do start reloading your own cartridges.