What procedure did you follow? If you simply requested a return the seller could have refused this, unless the lisitng stated that they accept returns. If you opened a case for "not as described" under eBay's money back guarantee this would have ensured a refund, with the seller also having to provide a prepaid return label.
The only way you could have lost a case would be if instead of explaining that the leg length was not as described you simply said the jeans didn't fit. As has been said, if nothing is said about the item not being as described eBay doesn't require sellers to refund buyers just because it doesn't fit them.
When asking questions it would help us to help you if you provide more information, including when and how you paid.