First, a nymph at the beginning and I argue intermediate stages of development for any player is "blown out of proportion" or "hyped up". In my opinion, the value of the elctrum to get other useful cards outweighs the value of any one nymph.
I can't find a single person that sold a nymph and didn't later regret it. Since most people will quickly believe that the a nymph is more valuable than the sell value of a nymph, it should be recommended that nymphs are not sold. That would make most people happier in the future. because they did not sell their nymph, which they think is more valuable than the sell value of a nymph.
Second, We grow and learn from our mistakes, if they wish to continue the game at a more developed pace: There are other ways to obtain that nymph. I argue that the luck of getting a nymph isn't "one time" nor is it exclusive to those who do it alone from the Oracle.
Other ways to obtain that nymph? On average, it takes more than a year to win a specific nymph one wants. This is evidenced by the fact that so few people have 1 of every nymph from non-manipulated spins.
I am agreeable though depending on the situation we cry out as well in our own counter lamentation: "DON"T SELL A NYMPH"
Take that as an guideline or piece of advice, not an ultimatum. It soley depends on the purpose/goal and how long the player
wishes to stay in the community: moreover, how active they wish to participate in "higher tournaments and events".
The problem is that players cannot ever make an accurate decision about whether or not they will play enough to value the nymph more than the electrum from selling it. In almost every case, they will wish they had the nymph in the future.
i.e.
if you're a collector and don't want to play anything outside the game itself, by all means keep the nymph.
if you're just trying to build a deck, and are testing out the community: It's not the end of the world selling a nymph, it may improve their chances now that they can afford more cards.
if you're planning to stay long term, want to collect, and experience everything the game has to offer(+forum event wise) keep the nymphs.
Players do not know whether or not they are going to stay long term. The small amount of electrum is not worth the risk of disappointing someone who will play more Elements in the future.
Nymphs shouldn't be called "rare/unsellable", but "rare/an investment" : Some players wish to "cash in early" others wait for the value of the nymph increases and becomes useful.
Most players that cash in early will later regret doing so. Since there is a way to prevent some of the players from cashing in early, it should be implemented.
I personally haven't seen a complete disadvantage of "not" having nymphs in my decks.
There are good decks that use nymphs. Not having nymphs would mean you are unable to use those decks; thus, you would be disadvantaged.
I actually kept my first nymph, and following ones because of the sell value.
At that point in the game, I wasn't a forum member, I had very few upgraded cards. So a sell value of 908 (or whatever it actually is) told me that the card was more valueable than anything I owned, or at least, more valueable than anu unupgraded card I owned - where Miracle apparantly was the best.
The giant notice saying not to sell the card will tell people it is a rare card. And a rare card that barely sells for anything probably won't be sold.
I think Nymphs should not be sellable at all. It should be a card of instigate value. Decreasing the value will prevent mistakes, but making Nymphs not sellable would eliminate mistakes. That's my 
on this topic.
This would be ideal. The only slight chance to this would be that nymphs could be sold if a player already had 12 nymphs of that Element. This would prevent the nymphs from taking up space, because an account can hold a limited amount of cards.
