Ebay coddles buyers.
A savvy buyer always has an out through ebay, if they carefully limit the reason for their return (or canceled transaction) to "I did not buy that, it was a mistake," or "it was different than I thought," etc.
It's a cost of selling on ebay. Even if ebay seller wanted (and adopted) policy of "no returns on CGC books," such a policy is practically meaningless since the buyer will find a way to return the book. Sellers still state in ebay listings "no returns" but practically all the bidders seem to have figured out that it's an unenforceable policy if they play it right (paypal protection, with perhaps some lying, is the ebay buyer's province).
Here it's basically the frontier with a few basic rules.
I agree with everything you've said...
It just doesn't address my question.
Okay, I'll try a different vector: why do dogs lick their balls?
Simple. Because they can.