I have to kind of agree with Mouse. I own one 
of 'real' Duo OP cels that was was sold by Forky 
recently and having seen the thing on eBay I 
wasn't the least bit worried. If I had the 
choice of buying either the real thing or the 
fan cel I'd take the real thing a hundred times 
over (despite the big difference in $).
For me I'm quite content with the fact that 
mine is a part of anime history whereas 
that's just a copy (and you could tell). Many 
people have famous prints in their homes but 
althought they're nice to look at, it can't 
compare with the satisfaction of owning the 
original.
My only concern would be if they were trying to 
pass them off as the real thing and I certainly 
agree that they should put 'Fan cel' in the 
description every time so nobody's under the 
illusion that they're getting a real bargain.