Your retaliatory reason for neg is exactly why sellers can no longer leave negative feedback for buyers.
You switched coins according to one buyer. Is that true? So why would you leave that person negative feedback? The other person said something was wrong with the item. Why would you leave a neg for that person? I know there are buyers who deserve negative feedback but just because you received it, that doesn't automatically mean the buyer deserves the same.
Non paying buyers deserve unpaid item strikes.