NEVER, EVER open a case with eBay unless you are the buyer. If he hadn't opened a return case, block him so he cannot buy again, then ignore all messages, unless they get abusive, if they do - ring eBay & report him.
If you have an item not as described, to open a case - go to your purchased item page.
Next to said item, on the right there is a clickable link which says return item. Click it, on the next page scroll down reasons to item not as described. They will have no choice but to accept the return AND issue you with a pre paid return label.
If the item is genuinely wrong, it makes absolutely no difference if the seller says they do not accept returns. This is the law, they have no choice. So, take some photos of the problem - wrong size / colour / item / broken & upload them on the case by clicking the little camera link. To be fair, if you are not techy, the easiest way is to put the photos on a computer / laptop / ipad & upload from there. Do this immediately you receive said item - although it is polite to inform the seller first.
Any decent seller worth their salt will be happy for you to do this. Ebay DOES NOT penalise a seller for cases open, ONLY for cases not dealt with. In fact, it is better for the seller that you open a case, because, otherwise when the item is returned, they do not get their final value fee back. I actually insist people open cases!
NEVER wait. If the seller is going to send you a new one / the right one etc, all well & good, they have 6 days to do so which is plenty long enough, if they don't escalate the case to eBay who will either
a) just give you your money back & you keep the item if it is low value
b) Issue you with a return label.
When you send something back, the seller has 6 days in which to start refunding you. If they don't ask eBay to step in.
If anyone needs any eBay help, feel free to message me, I make my living as a Business seller & am always happy to help & NO I don't expect you to buy anything :)