How to prove a steam account recovery?

I traded someone for a steam account a long time ago and the guy recovered it like a week afterwards I recall.

I have thread as proof that I bought his steam account, and he posted on my thread that he sold me one.

However I have no clue how I can possibly prove that the steam account has been recovered by him? You can move this if needed so other people could give me advice, I just couldn't think of the right place to put this.

Ty.

 

Pm XceL.