My game account was hacked and sold on this website. How can I report the person who did this?

[Client:77795473]

Hello everyone, and sorry for not speaking Polish but I barely managed to do even this using google translate. I have not played a video game (league of legends) for about 3 years, but recently I received some e-mails that there was activity on my account. I changed the password and logged in to see that indeed my account was being used. I contacted the people in the friend list inside the game and they confirmed that their friend has bought an account from a seller on this website. The seller has claimed that they have ownership of the account and that it was 100% legal while the account was stolen. I have managed to contact the buyer and he feels scammed as well because I was able to reset password and refuse him access to the account he bought because I am the original owner. He has provided me the name of the seller.

How can the seller face consequences for cheating on this website? I have all the proof necessary to do this and possibly to make moderators remove him from the website. 

Thanks for understanding and I hope that I have posted in the right place, because it is very hard to navigate website using only google translate.

[nat_not]

@Client:77795473 You are not our client, which makes this issue complicated. It is the buyer who should contact us if there are problems with the ordered item. Use this form to report the problem: https://allegro.pl/pomoc/kontakt?kategoria=65a29468-5e40-4672-a8d6-e0a1f77e6d3a&subjectId=5d429015-5... We'll try to help you.