the game studio that developed them must still be in business, it can't have sold out to another company

This is great, I am going to adopt this. I have not checked for this in past, but it is very important.

Ad. second hand markets - some Indies wrote that they would prefer people to pirate their games over buying on G2A or similar sites because if sellers got keys via stolen credit cards it actually costs developers money on charge back fees, which Valve demands from devs in full. Not sure if its still true, but it made me not wanna buy anything on key reselling sites.