I always clean up my scans.

I find that my scanner can't reproduce colors 
well enough to give anyone a good feeling for how 
vibrant the cels look.  Sometimes -- my favorite 
is my Kusinagi cel -- it's dark enough that 
without serious work you can't actually SEE the 
great detail on the cel!

And what's the point of having a gallery? For me, 
it's to be able to enjoy my cels without 
subjecting them to fingerprints, cat hair, 
children, and harmful solar radiation.

SALES are another issue, as people have said.  I 
would not sell my cels of Heero Yui or Dark 
Schneider, for example, without rescanning to 
show that there is a hair-tip missing on each.  
That's ok, though, because (crossing fingers, 
knocking on laminate) I'm not planning on selling 
them any time soon...