You just use the eBay money back guarantee, it is on every listing, top right.
You open a not as described case, to get a full refund. Seller either refunds or sends a return label, and refunds after retiring.
You do not need to contact eBay, or the seller, there is no point in arguing, when the method of dealing with the problem is so simple.
ONLY if the seller does nothing once you have opened a case, after three days, you escalate to eBay who take over and refund, and seller gets penalised.