Whether it's $1000 or $2500, responding her to "inform" her that her price is slightly high, is different than telling her that she's being mocked behind her back, that she'll maybe find a "sucker," and that she "really doesn't have a clue about mopeds."
You weren't messaging to help this lady sell her bike, you were admonishing and criticizing her for her high price because you felt it should be cheaper and that she didn't know what she was selling.