If you sold back spells for training points, unfortunately, it was an all-or-nothing deal. For trained spells you wanted to keep, you have to go back to every trainer and train those spells back again.
There are astral trainers in Celestia, Zafaria, Azteca, and in the Arcanum.
In Celestia, go to the Floating Lands and inside the Stellarium and Lunarium for access to all the spells.
In Zafaria, the sun and star trainers are inside Zamunda, and at the Drum Jungle dock. I don't think there were any moon spells there.
You can't sell your spells. There's no such option in the game.
However, if you accidentally removed your spells by using the buy back your training points option from the local registrar in Golem Court, Wizard City, you simply return to the school trainer to re-learn any "lost" spells.
I highly doubt this was what you did though as if doing this isn't something you'd forget, especially since crowns in involved.
So, I'm guessing the spells are most likely still in your spell book - Check the relevant taps under the spells icon.
However, if you're referring to Treasure Cards (which you can sell), you'll simply have to:
Alia Misthaven
