Another reason is that it was signed in sharpie... Sharpies weren't made til 1964

Its a fake...

Thank you very much for this advice! I won't buy it ;)
If you have some ideas where I could a real one...!

They are lying to you. But here's a few reasons why this is fake.
First you can't purchase an authentic James Dean autograph for $225,-
Second, A German poster - Rebel without a cause was released in Germany on March 30th 1956. James Dean died in September 1955. I doubt there was any European movie poster even printed for Rebel before he died. That poster is not original, and if it was it is now ruined with a fake autograph.
Third, that signature looks nothing like James Dean's signature. The slant is wrong, the letters are wrong, all of them, but most notably the J and D. And no his signature didn't change. Only one movie was released while he was alive, so not a lot of time for someone to change their autograph, and all examples I've seen from that period are consistently the same. The only time his signature was different was when he was a teenager(like in some signed high school yearbooks).
I can go into a lot more specifics, but I think this is enough. Trust me on this, I have done a lot of research and have enough experience with James Dean's autograph to know the difference between the real deal and this.
Don't buy it!

Thanks for the answer but I could you be sure, the poster provides from http://www.neautograph.com/inc/sdetail/3653 and they told me that it was signed during the filming of one of him movie and that his signature could change . What to think about it?

Oh no. That's a total fake.

Hello!
I've bought this article on an american website famous for autograph with a certificate of authenticity for 220$. The fact is that it doesn't absolutely look like the usual James Dean's signature!
Is anyone a good "knower" (sorry for the bad english) in James Dean autograph?
Thank you a lot for your answer