Why do witches get special treatment in Majo no Tabitabi?
My theory is that those brooches can only be created by witches. Thus anyone with a brooch is a certified witch with another witch backing. Witches are not rare from what I have seen, but because witches are better at magic than the average person I reckon they are generally held in high regard.
About the buying of brooches from witches (considering that only they can create them). I suppose some witches who are evil (they're still humans) would sell a brooch, but I have not seen any evidence of witches who would do that.