Should the nymph sell value be reduced ?
I would vote NO, the sell value should show the rarity of a card.
There should be a stronger warning. One option could be a question mark for rarity in the oracle: The text next to the ? shows the rarity of the card, the ? explains the concept of rarity.
What do I do if I have 6 upped nymphs and 6 unupped in an element and the Oracle gives me another one?
Then, this makes you very special. That´s why I would still keep the nymph. It could happen in a few years, but then, the game could be totally different.