I think you're aiming for more of an ethical 
discussion than a legal one.

Is it ethical to reproduce cels as fan cels?  I 
think it depends on the context.  The artist 
certainly seems to mean no harm, and her prices 
are very reasonable.  IMO, it's a far cry from 
the activity known as counterfeiting, since there 
seems to be no intent to deceive on her part.

I personally would feel rather flattered and 
proud to own an original cel that someone decided 
to have a fan cel made of.  I'm always outraged, 
however, to see fan cels passed off as production 
cels.

There seems to be a fine line.  Is it fair to ask 
the fanartist to somehow mar their works with 
some sort of indication that the cel is a 
fancel?  I'd be a bit offended to be an artist 
asked to do that, but it seems to be an easy 
solution.  How far is someone willing to bend to 
thwart scammers?  Isn't it enough for the buyer 
to ask to see a full scan of the cel, and check 
for sequence numbers?

Just some thoughts.  ^^;