Just two simple pieces of advice:
1) If something is too good to be true then it most probably is - I have seen people on sites post their latest acquisition and it is an Ausweis for Reinhard Heydrich (fake of course). Ask yourself, what are the chances that something like that to such a personality would just turn up on the internet market or a militaria fair stall for a couple of hundred £££/Euros/$$$?
2) Trust your gut instinct - not exactly scientific but it will save you money and hassle more times than not. If you see something and you get that little feeling niggling at you then walk away.