That depends. Some sellers refund and not require you to send it back because the cost of the return label might not be worth it. Cheaper for them to refund and let you keep it.
In other cases the seller meant to refund when it's returned but they hit refund because they intend to refund but they don't realize it refunds immediately.
And some sellers don't agree to the refund so they ask eBay to decide but they don't realize when eBay has to decide then the buyer might end up with the refund and they're told they do not have to return it.
If eBay has told you that you don't have to return it you can keep it.
If the seller provides a return label you should send it back because it's not right to get the item and your money back (unless it was destroyed).
