Seller learns why to send cancel when buyer asks for cancel

A buyer recently purchased $300+ worth of items from me. He requested a 15% discount after the items
had ended and prior to paying. I declined. He said to just do a mutual cancelation then. I refused and
said I'd file NPB alerts to get my fees back. So (after several insulting emails), he pays and then says
he will just return the items once he receives them to get his money back and leave me negatives
for all the items (100+ items). 

I contacted ebay, and spent nearly an hour on the phone with them. They read the string of emails,
acknowledged that it was extortion, but said they couldn't do anything to help me out and they wouldn't
remove the negatives if he leaves them.

Note that seller is guaranteed protection from any neg when buyer requests the cancel, seller sends the cancel, and buyer declines the cancel.

CS will also refund the fees.