First contact the seller and politely explain the problem.
He can:
- Tell you to keep/dispose of the unwanted item and refund you.
- Tell you to return the unwanted item and refund you the original payment and your return shipping cost.
- Tell you to return the unwanted item with the Return Shipping Label he sends you and refund you.
- Tell you to keep/dispose of the unwanted item and send a replacement.
The first is the best for you, the last is probably the best for him.
If you both agree, and he follows through, great.
If he makes a problem of this, go to the eBay Resolution Centre at the bottom of this page and open an Item Not As Described dispute. Since you had already communicated with the seller, you can skip that step and go immediately to the Claim.
EBay will tell him to send you a Return Shipping Label to return the unwanted item and refund you.
If he doesn't eBay will refund you and go after him for their money.
Politely-- Captain Malcolm Reynolds