Again, like the honey articles talk page, its fan speuclation that you can sell the milk but its the most we know, so I believe it should stay.
According to the dev interview, milk apparently has some secret use to unlock something else. See 
Use for Milk
I've heard mention of cat and dog pinatas... so I'm guessing that Milk is a requirement to attract cat pinatas.
--Jimmcq 20:04, 20 August 2006 (MDT)
About cats and dogs
On the subect of cats and dogs, I think instead of a flat out cat or dog, how about something in that family. For example, a lion or tiger for cats and for dogs, wolves. (all predators)
The description for the 'Harvester' achievement states that it can be achieved by collecting produce from a Buzzlegum, Moozipan or Goobaa. I've tried it several times with the Moozipan, but it didn't work.
Has anyone else had this broke?
You can unlock it by buying a jar of honey. --Eiyuu Kou 18:35, 8 March 2007 (EST)
Milk and young pinatas
I did some research and milk allows young pinatas to grow faster. Try having a Cluckles to hatch eggs and milk to feed youth for ultimate growth. --Eiyuu Kou 18:35, 8 March 2007 (EST)