The Crystal Werewolf is the perfect example for prices going up and down like crazy. I got my first one about four days ago, and sold it for 38 cents. Then I got two more the next day, which I put up to sale for 25 each, the lowest bid at the time. Yesterday I checked, seeing that I still had both wolves, and no wonder: the price was down to a dime! Still, I didn't bother canceling my bid, and by this morning they were both gone. The current price is 23 cents, by the way. Makes me think I should have bought a ten-cent werewolf... Even if I got stuck with it, it would have brought me a profit of 28 cents!
These last few days I've been actually more excited about trading cards than playing them, making me wish they had charts for them like on Poloniex.