Start by asking the seller for the money. Politely.
Do NOT leave feedback (yet). Do NOT mention either Feedback or Detailed Seller Ratings (which are more important than FB).
Those are your 'tools' and if he is smart he will realize that you have a lot of power in this situation.
Politely.--- Captain Malcolm Reynolds
https://www.youtube.com/watch?v=jZ3k2xA1Wcw