An irresponsible buyer

[BitBomber]

I met one such buyer.
The first time he placed an order, the package was returned because no one received the package. The postman failed to deliver it twice.
Then, he placed second order and bought the same product, a gaming mouse.When he received the mouse, he told me that there was a delay in the mouse so he applied for a refund.We analyzed the video he shot and found that he connected the mouse via a USB cable.In this connection mode, there can be no delay in the mouse.When we were looking for the cause of the problem, he insisted on returning the goods and threatened me to upload videos on YouTube and Tik Tok.So, I sent the video to the manufacturer of the mouse. After analysis, we found that he directly attached the extra skates to the bottom of the mouse without removing the original one, which caused the mouse to not be used normally.We pointed out his problems.I have rejected his return application.He still insists on returning the mouse and intends to send it out.
I am a Chinese seller, selling mouse and keyboard on allegro.I have very few orders.For his first order, I lost PLN 291.00.The logistics company told me that according to Polish law, I cannot sell the mouse to others and the mouse will be discarded.For his second order, I am about to lose PLN 291.00.
I sent an email to Allegro, hoping to get help, but I haven't received a reply yet.
I want to know how, as a seller, I should reduce my losses.
Besides, he kept threatening me, which made me very angry. I told him I would sue him, but he mocked me for not being able to do it :(，which make feel worse.

[NipponShorty]

Pozostaje iść do sądu, niech rozstrzygnie was spór kto kogo obraził.

[la_nika]

Hello @BitBomber! Your email has been forwarded to the appropriate department, so I must ask you to be patient.
However, if I understand correctly, the buyer has filed a complaint, you can accept or reject it. Based on the information you have provided here (verification with the manufacturer, rejection of the first complaint), you have appropriate arguments to reject the complaint of the second order. You have already replied in the ongoing discussion, if the buyer writes again, inform him that you reject the complaint and use all of the above arguments. 
If you find messages from a buyer that you find offensive, you can report them to us using the form. Please send screenshots of the messages you received.