Unfair negative reviews due to buyer user errors – How do you handle them?

Happened to me more than once. In the past I used to contact Cgtrader support, explained the situation and the unjustified review got removed. Today support seems to work slower based on other users feedback, however I have not tested it myself.

It's good idea to post disclaimers in the description and at at least one image so it is completely clear for the Cgtrader team the client was in the wrong. Such disclaimer should clearly state what the product isn't for. It won't guard you from user error since some buyers don't read the description or have no idea what they are doing, since some of them are absolute beginners to 3d.

In your specific case it would be also good to include some kind of how-to-use note.

Mail the site support; it happened to me once. mailed support with the solution and proof that the model is up to standards.
They shall remove the negative review

Happened to me recently as well. Buyer buys 3d model -> tries to print it -> fails -> writes negative review. Thing is, the model was not meant to be 3d printed (not printable asset), model inner object separation was visible in photos and it's not my problem that clients slicer settings were in correct xD I just left the review hanging. Other platforms seems to handle such problems better