It's not outrageous to think they would start if 
there was a market though. Even individuals might 
start putting cels that are very hard to tell if 
they are real production cels or not, they might 
go as far to where you can't tell unless you are 
in person to see the cel. I see this starting to 
happen as more and more anime uses digital 
painting and a demand is there for cels from the 
series. Most will know they are fan cels because 
no real production cels exist, but people might 
still buy them for this same reason. Putting 
fancels on ebay is wrong in and of itself. Any 
respectable fancel artist shouldn't do this. They 
are making money off from someone else's work. I 
don't see it now as a major problem but when cels 
become more and more scarce 10 years down the 
road, there might be more people making cels from 
older series. Nuriko's death scene has been made 
into a fancel a few times at least from what I've 
seen on ebay. The person that owns the original 
cel is probably pissed as they paid a lot for it 
and now others have the exact same cel, only his 
is the real one. I know I would be if I paid a 
ton of money for a cel only to have it fancelled 
later on.