With SM cels its all about how much you want to 
pay when that nice cel comes around.  Right now 
Chirty has a nice Mercury cel for sale on her 
site.

Mercury cels can be hard to find, since I think I 
a lot of people want cels of her!
THis is a very nice quality Ami cel and to find 
that nice of a Mercury cel could take a while.

AS for tradeable, I am not sure you would have to 
find a gallery site, that liked Sailor Moon and 
these others series.