It all depends. I do think splitting up cels and 
sketches and selling them off separatly is wrong. 
But if you have no plans of ever selling the 
sells, and if the sketches are only sitting 
behind your cels doing nothing (i.e you have no 
attachment to them whatsoever) and someone else 
truly appreciates them, then I think there is 
nothing wrong with selling them. Just my 2 yen ^_^

Ja!

-Misha-chan