BitSynCom is a USA corporation, incorporated in the state of Nevada.
I don't see that entity on the website. Possibly that was deliberate. Is it listed once you do an account or order with them? You can't lien or attach BTC but does the company have any assets in the US that could be frozen by lis pendens? Since yufu is in the US the easiest route would be to serve him personally. He might have been deliberate in publicly stating he doesn't control the BTC sent in but you might be able to depose him under oath to get some answers but the process is very long for that if he fights it.
You should probably read the current render of avalon-asics.com before posting such dribble. It clearly says copyright Bitsyncom LLC. Not to mention, the waybackmachine records of the avalon-asics website during those orders. It's all very clear that Yifu and Avalon were operating as a Nevada-based domestic LLC with its managing member (Yifu) residing in Brooklyn.