the easiest way to work out whether a motorcycle is likely to be legit (legally imported with taxes and duties paid) and therefore likely to be legally plated etc is to look at the prices asked. If the price is roughly 200% of what the same bike is new in most overseas markets e.g. USA, then you are likely looking at a genuinely imported bike. You can apply that to a second hand bike which if it is legally imported 100% will hold it's price remarkably well, up until it's about one or two years away from it's scrapage date, e.g. when it must be permanently removed from Chinese public roads and plates surrendered etc. So even a used second hand bike legally imported will cost you more than the same bike new in USA. jszmtoto are not selling legally imported bikes based on the prices I'm seeing in their ads. As for papers, they will tell you anything to get you to part with your money and then you will be on your own. Good luck with that...
You want to buy legal? Then be prepared to spend some money on anything 600cc and up. There are lots of options... <250cc and a reasonable number of options for >250cc but the latter will cost you. If you want to buy anything else then there are few if any trick ways... to be 100% legal/legit. And there are almost no ways to try and make a illegally imported bike legal, legally e.g. pay some taxes etc. after the fact.