If the cel is stuck to a sketch or paper you can
easily remove it w/o damage.  If it has a
background it is very common for it to be stuck :(
 That doesn't bother me so much unless the paint
from the background has bled through into the cel
paint.

Line fading does bother me and unless it is a
wishlist cel I wouldn't buy one with bad
fading/condition.  If so I would probably get a
line correction layer made.  It would, however,
lower the price I'd be willing to pay.