take in mind that every NPC has a particular taste, so first talk to them and sell the milk to those who have a liking for the milk you have (What people likes, is based on the Trending of that day, in your nest you will find a paper on the wall about it)
EX: You did BunnyxKelpie milk, now you interact with a NPC, if they say something about cows or any other fapet but Bunny or Kelpie, they will not buy the milk.
If they did say something about Bunnies or Kelpies, they will buy it