Not necessarily. Some people don't like the big oversized cases that come with the glasses and prefer to use one of their own cases.
Then there are people who buy sunnies at TJ Maxx and they don't have the Coach cases. So those people might shop on ebay for a case.
Personally, I don't see a problem with someone selling something they cannot use. As long as the case is authentic (and there are plenty of fakes out there), it's theirs to do with as they please.
(I admit to feeling differently about sellers who remove hangtags and charms from bags and sell them separately. I see that as being unethical, especially when many of those sellers are also selling the bags separately, sans their hangtags.)