Why can't they sell Master Balls in the Pokemon Games?

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I just wish they would sell Masterballs even if it was a great price like 10,000 you see its frustrating how they cant sell them because then you only get 1 and thats only for your legendary from the game at least if there was this hidden place in the game like a special cave or something where they see them for a high price. what do you think should they sell them or not?

in FRLG if raikou or entei used roar then yes they are gone permanently

well....that sucks...give more master balls to me then

And did they have a high chance of using roar?