Anime (cels included) is a fad/new for a lot of 
people. They probably don't search the intenet 
for cels very often and don't know that 
watermarks are used to protected important images.

As for the drawing, I would keep it. *evil grin* 
                     
I rarely sell my Sailor Moon cels; but when I do, 
I usually keep the sketch. Drawings don't seem to 
interest many people; but I love them and buy 
them with or without their cel siblings. 

If you decide to keep the drawings, make sure the 
price of the cel reflects it. Then you know it'll 
be taken care of.

Don't feel too bad! At least you were spared the 
evil: "What?! $200 for painted plastic?" phrase.