Sellers get a larger margin on eBay. You can’t get perfect but you can get better. The return/exchange policy is a small tradeoff for not supporting one of the most evil corporations in existence. It’s also kind of a selfish thing to care about, most of Amazon’s returns are just trashed into the landfill so it’s not like it’s green or something.
Those are reasons a return policy is nice to have. Yes, it’s not a great idea to shop for clothes on eBay, so that can be an exception? The point is to use the giants less, it doesn’t have to be all or nothing.