Interesting, I´m constantly breaking the rules:
Rule of Thumb Number One: Never sell upgraded cards unless you have more than six copies, and never sell unupgraded cards unless you have over 12 copies total (unupped and upped).
Partially agree. If a FG gives you a tower, the single tower upgrade is close to useless. There are also other upgrades, which are close to useless (e.g. Stone Skin).
Invested into another good card (e.g Supernova, RoL) or a better unupped card pool, the money is invested much better. My Rule of Thumb is: "Upgrade cards, not decks" because decks aren´t made for eternity and "The upgrade of a normal card is usually better than a tower upgrade" - maybe with the exception of Quantum Towers , they are by far the most used towers.
For practical reasons, I keep exactly 6 copies of all unupped bazaar cards (except pillars and pendulums) and sell them, if I have more.
Rule of Thumb Number Two: Never buy or upgrade cards unless it's necessary.
I see no reasons to keep too much money (although some thousands are good) on hand, because the money in the treasure chest does nothing. We have no bank, where "money can work". I think "necessary" means the upgrades you need so that the deck strategy can work.
I´d like to add a rule no 3.
Rule of Thumb No3:
Don´t sell nymphs and it´s better , not to upgrade them.
A sold nymph never comes back, nymphs make your account unique and upped nymphs can´t be used everywhere.