I used to do sell them and I could. However, burning off my excessive amount of coins on Lv.5 Manyus saves me trips to the Secret Gardens, which then becomes stamina for me to farm Life Crystals with, which is a reasonable tradeoff.

Regardless of what I choose, I'm still drowning in an obscene amount of resources better off helping other players. The only way I'd be able to burn through them all right now is if I got maybe 5 or 10 new Knights all at once.