"I say that it is clearly one of the above because of what I know with certainty now that I received one: First, it comes in a Jack Wolfskin plastic bag. Second, it says Jack Wolfskin in two places on the actual garment. And Third, the tag that came on the jacket is the official red and back Jack Wolfskin tag stating, "At home outdoors in wind and rain - JACK WOLFSKIN OUTDOOR APPAREL is your reliable partner in every situation........" in German, English, French, and Chinese. The tag even points to their official website of www.jack-wolfskin.com."
Counterfeits can be quite good. I bought a TNF pack that had the logo cast in all the hardware. The packaging and labels are easier to knock off than the product and might even come off the same printing line. The guys who do the sewing already know the printers--- might be family too. A lot of this stuff is sold on the side of the road and market squares in other countries and the packaging adds to the cache. As you have found out, there are many sizing standards worldwide.
This isn't new and it is widespread. We are generally insulated from these products by our government, but eBay creates a lot of holes, I'm sure.
Best of luck!