already been prepared.The inclusion of market refers to the fact that this person’s buying behaviour (from the market’s perspective) is like a vegetarian.
Person 1: Did you just have a meat pie? I thought you didn’t want to have meat at the restaurant last week?
Person 2: I don’t buy meat, but if it’s already been purchased I’m ok with having it.
Person 1: Oh! So you’re like a Market Vegetarian?
Person 2: Exactly
Person 1: What a hipster