I've noticed the "Sell" button below the girls disappearing at random when I play. After testing it out it only ever seems to happen after I "place" an animal from the barn, it's never when I buy directly from the store. Sometimes it kicks in right after I place one of em, sometimes I need to sell one and then it kicks in... I couldn't pinpoint exactly what caused it.
Not sure if you were aware of this, and not sure if this is the best place to report it, but wanted to put it out there for ya one way or another!