File dispute with reason being that the goods are substantially not as described.
Refuse to pay anything out of pocket for their mistake, such as return shipping. This includes paying ahead of time and having them reimburse you later, unless you have some sort of protection like escrow or a guarantee from the facilitator (assuming ebay here). Make sure that you unquestionably demand this.
If he sends you a return shipping label or otherwise pays for it in a secure manner that ensures you'll safely get your money back, you return it. Don't be a dick.
Optionally also leave feedback that's actually useful for others. You shouldn't someone 1 star if they handle the situation professionally, but you also shouldn't cover up their carelessness with glowing 5 star feedback.