Welcome redwingjamie, I hope you enjoy the game and the forums!
And as for your question, don't sell Nymph cards, no matter how useless they may seem if you ask me. I remember selling one or two back when I thought I needed it. Now? I can get more electrum with ease (don't even ask how much spare electrum I have, but 908 electrum is a joke compared to it), but the nymph is gone forever.
No amount of electrum will bring that specific nymph back, although given enough luck you'd get new ones, eventually.....