Yes, Cubase 12 is fully compatible with Big Sur, Monterey and Ventura. But I can run Cubase 12 pro on a Mac book pro early 2011 running a patched Catalina (dosdude), so, Big Sur will be fine.
You have to be very careful buying second hand.
If the seller has redeemed the Cubase Artist 9 code, deregisters the USB eLicenser containing the licence from their My Steinberg account and hands you that eLicenser, then you’re good to go - you will have a licence for Cubase Artist 9.
If the Cubase Artist 9 code has not been redeemed, then it will redeem for the latest version - you’ll get a Cubase Artist 12 licence that is useless to you, as the only version of Cubase you will be able to use is Cubase 12 which does not run on your Mac.