Well.. it's not that the place selling it would 
be the party who was directly involved in the 
cel "theft"  ...  lots of people who were 
involved in the animation process most likely 
took some cels home. They later sold those cels, 
and so they got into circulation.  :)   I have 
seen a lot of *very* reputable dealers who have 
had cels from this - Anime Taro & Anime Chaos I 
believe both had them.  And Stephen and Nick are 
both fantastic to work with - and I enjoyed being 
able to see the cels (even if I couldn't afford 
to buy them at the time. ^^)

Just my thoughts :)