EVA cels were not destroyed. They are kept by
the studio because the company holding the
rights for all produces related to the anime 
doesn't want them to be sold.

EVA fan cels are probably a worse case : 
GAINAX/Bandai/SEGA asked, in the past, people
who had real cels to remove them from their
website. And they threatened people doing
fancels...

So whatever collectors/dealers think about
fancels, it's probably not a good idee to do
Eva ones.

Cheers,
Yann Stettler