The Mystery Egg cannot be purchased in the FarmVille Market directly and it does not count towards the They of Mystery Ribbon.
Now what everyone wants to know, what’s inside the Mystery Egg?!
What did you get inside your (redeemable) Mystery EGG?
::: Spoiler Alert :::
The following are confirmed Mystery Egg Prizes:
Double Avatar Size
Small Can of Fuel
FarmVille Freak DarkFarmer's Double Avatar Noitce
FarmVille Freak DarkFarmer's Double Avatar in Gift
FarmVille Freak Christopher's Double Avatar on Farm