Always keep all non-buyable cards (heck I'm even keeping them over 12, you never know if in a few years or something you can use more than 6 of a card
- but I don't recommend that).
As for upgraded-non-upgradable-buyable-cards: I sold them until I finished rol/hope (my false god farmer). Now I keep them.
PS: I found from AI3 owl's eye the least rare rare-card
. Morning glory I still have to get from AI3.