Hi Gajavaktra
I think, you just not need to worry.
If a buyer leaves u a negative feedback, but he did not pay. eBay has its policy for it.
Just open an unpaid item case against the buyer and close the case after 4-5 days, Automatically that negative feedback will get removed.
NOTE:
If buyer asks you to cancel transaction after leaving negative feedback, do not do that. Because if you do that, that will not remove your negative feedback.
Have a great day ahead.
Best Regards
Aman
SmartPhoneForEver