1. Decide if it's really a battle you want to fight, or if you can just chalk it up to a lesson learned for the future. Sometimes it's just not worth the hassle even if the seller is in the wrong.
2. If yes, don't immediately file a dispute, just contact the seller and ask for a resolution. Be specific about what you want from him/her. In general, a straight return is probably the best resolution, as it's pretty messy to ask for a partial refund.
3. If seller doesn't help you, then file a dispute.