Acknowledge the frustration
Thank them for raising it and show you take reviews seriously.
Check the review against the rules
Look for abuse, personal data or off-topic content.
Say no to fair negative reviews
Explain that honest reviews stay, even when they are negative.
Offer the right options
Suggest a public reply or a way to resolve the issue with the buyer.
Remove what breaks the rules
If the review breaks policy, remove it and tell the seller why.