It sounds like you are accepting returns. But you should have the option not to accept them.
I am sorry to hear that. Seems like the best thing you can do is to diversify and not put all of your eggs into a single basket. There are marketplaces where you can get scammed easily, where you simply need to accept this as the cost of doing business.
We take returns, then when we get a negative it becomes a libel issue with the buyer. They have no reason for the attack when they have/had the opportunity to send it back and zero damage is done to the buyer.
Yes, I know that on eBay you can have an unfair feedback removed by calling eBay.
I have been selling on eBay since I was 12 and I remember spending school breaks calling them, and then persuading the representative over the phone to remove it.