If you haven't already done so then contact eBay Customer Services and request a phone call using their callback service. eBay seem to have messed up the option to request a phone call, the end result being that it's no longer as straightforward as it was, but if you click on the link shown at both the top and the bottom of this page that says "Help & Contact" a new page will appear on the screen. Click on the Selling option and when the next page loads up click on the option that says "Appeal A Defect." A new screen should then load up which gives you the option to choose to have somebody call you. Click on that link and you ought to receive a phone call from an eBay representative.
Once the eBay representative calls you explain the problem to the eBay representative, in particular pointing out the fact that the buyer contacted you and made it clear that he had no intention of paying for the price that he'd won the auction at, and had told you that he would pay you less for the final sale price. Make sure that eBay check out the e-mail that the buyer sent you. Once they do say something to the eBay representative along the lines of "The buyer clearly stated in his message that he was not going to pay the price that he won the item for, so I cancelled the sale given that he no longer appeared to be willing to pay for it and he left me a negative feedback." If necessary argue the case that given the fact that the buyer refused to pay for the item he should not have been allowed to leave you any feedback whatsoever, and request that the feedback be removed on that basis. If you get through to an eBay representative with an ounce of sense then once they see the message from the buyer that makes it clear that he had no intention of paying you eBay ought to remove the feedback and the associated defect. If that is indeed the case then the buyer's negative comment will not have caused any damage to your account, as your account will be restored back to its 100% positive status if you win the argument and eBay agree to your request for removal of the non-payer's feedback.
If you have not already done so add the buyer's User ID to your Blocked Bidders List. If you're unfamiliar with the process click on this link https://offer.ebay.co.uk/ws/ebay/isapi.dll?bidderblocklogin and a new page with a big white box on it will appear on the screen. Add the timewaster's User ID to that white box, then click on the Save button beneath it. Once you have done that the buyer in question will be unable to purchase anything else that you list on eBay at any point in the future, in which case he won't be able to cause you any further aggravation in relation to anything else that you sell on eBay.
