I've seen some Captain Planet cels cross eBay,
but I've never seen any of the other series 
(or, rather, never looked).

I saw one small image of Captain Planet
himself (was a decent shot but he was probably 
only 4-5 inches tall and on the side/corner)
that went for about $25.  I saw one backgrounded
cel of Hoggish Greedly and his henchman that
went for $9-$13, and I saw one backgrounded cel
of all of the Planeteers running (the cel figures
were very small, full-body images) face-front 
that went for $15-$20.

I wish I would've saved the images!  :P  These
auctions were on eBay within the last 9-12 months.

The cels seem to be out there for this series
(although not many) but not really in demand...
same for another network-buddy that played 
around the same time (Pirates of Dark Water) 
which are floating around and not really going
for much.

Hope this helps!

Kim
aka Lady Zatara
(who wishes she would save pics more often so 
she'd have visuals to go with her messages :P)