...ANG is infamous for studio made repros. So 
they're not really fan cels, but the market IS 
flooded with official cels that aren't actually 
production cels. These cels are often marked the 
same way the production cel would be (A-4, etc), 
and look almost exactly the same, save for some 
tiny errors here and ther that might give it 
away. Sometimes they're flawless (so just because 
the cel "looks good" doesn't mean jack, in the 
end of it).

Because of this, sometimes even the seller does 
not know it is a repro. So the seller could be 
telling what he thinks is the honest truth 
without knowing.

Really, all I can say is, that with AMG cels, 
especially the more primo shots, I would be 
careful. Belldandy has a particular problem with 
the fact that she seems to be the majority of 
repros.

However, if you are looking for your cels on 
DVD's, I would not do that, as the compression 
removes frames and so the exact frame your cel is 
on may not always be there.

Hope this helps!

-Misha