So why even have reviews in the first place. I think if there is a problem, the buyer should contact Ebay privately, and then Ebay can work out the issues between the buyer and seller, and keep things between the two parties.
You make a valid point. Ebay needs to have reviews so people will feel confident in bidding. Suits like this will chill the willingness of people to give truthful feedback. Perhaps Ebay needs to have as part of its terms of service that a suit like this violates its terms of service. All disputes about feedback should go through Ebay. That could probably be enforced as a type of alternative dispute resolution agreement. Then this lawyer would be out of luck.
Great idea ... except that eBay doesn't want to do squat, except collect their fees. eBay doesn't want to do any actual work ...