You did everything right imho, seller was clearly playing games (and would've kept doing so if you'd agreed to his obnoxious return terms). If it were me, and eBay refunded me and said I get to keep the items, I would keep the items. But I can be a jerk
If he'll pay for the shipping and you want to send the books back to him, that would certainly be some positive karma for you. But then you also risk him playing the "books x, y, and z were damaged further when you shipped them back, now you owe me compensation!" and him incessantly harassing you further.