Well, this is my advice. You could be stubborn about this and the buyer could go ahead, accept it and give you a bad review like 1* maybe. Then what happens? You now have a bad ratings and bad reviews that’s going to affect your account and now you wont be able to make anymore sales on this account. Like many people just told you, its best to move on and keep your good ratings. Many buyers don’t read. As a new seller, its not worth it. An old seller with lots of reviews can do this and it won’t affect them much, but you, it will affect you. Another thing you can do is contact support and let them settle this but i know what they are going to do. They most likely will go with the buyer. So take my advice. Accept the cancellation and move on.