Hiya. As both sets were sold as one listing...... if you open a case under Ebays Money back guarantee then you will be required to return both sets so the seller is correct in asking that both sets be returned for a refund.
A lot of sellers including myself will not deal with part refunds.
As for the abuse...that is not acceptable and you can report the seller to Ebay who can read the message if it was sent by Ebay messages and contact the seller.
Hope this helps.