The buyer inspected, deemed the goods to be as described, and took them home.
At home they noticed that the goods were no longer as described. How exactly can they prove that they didn't do the damage? Perhaps they dropped the monitor whilst transporting it?
The issue of the Trademe account is a red herring and is not an issue unless/if Trademe starts asking questions.
I frankly cannot understand the many responses here who seem to suggest that you need to pay the buyer back even though you are a private seller, the Consumer Guarantees Act does not apply, the buyer inspected the items and thought they were fine, and only on return with the goods to their house suggested that they weren't.