You might find some of the information below 
interesting because I've had a fair amount of 
experience related to your problem. I have 
purchased an AMG cel from Nipponya, collected 
dozens of AMG cels at auctions, dealers, and 
private collectors, and have seen a fair number 
of "fakes". 

Regarding fakes, they're not usually fake as 
in "not made by AIC") because they were 
distributed by AIC to being with. I call them 
repros, even though I believe most were used for 
tests as someone else stated earlier. 

The best way I've found to check for repros is to 
simply overlay the picture of the cels and a DVD  
cap with Photoshop. Differences can be spotted 
quickly and Photoshop can even convert an angled 
photograph back to a flat undistorted image that 
can be overlaid perfectly with DVD cap. Then, 
simply toggle between the images to spot 
differences. 

The good news is that Nipponya does not always 
sell repros. The BD cel I purchased from him is 
the real thing down to the flaws (cel painting in 
the earlier episodes, especially 3 was pretty 
sloppy). I received a similar email when I won my 
cel, which turned out to be real. However, I had 
done a fair amount of checking between the cel 
and screen cap before bidding.  

The bad news is that this isn't always the case. 
I did spot some cels that Nipponya sold which 
were obviously not production. Very often, 
sellers don't know whether they have a repro or 
not so I'll give him the benefit of the doubt. 
Also, many AMG OVA collectors would rather not 
know that it's not real because they paid good 
money for their cels.

So the good news is that if you can spot 
significant differences with the screen cap 
(highlights, wrong colors in some places, 
missing/extra paint, flaws that don't match), 
then it was probably a non-production cel sold at 
the cons for cheap. The reason why it's good news 
is that if you spot it now, you caught it before 
you paid. The situation could definitely be 
worse :)

- Chi