Okay, I'm a newbie so...I won't know why its 
taboo BUT my opinion is that * IF * you have 
the cel with a matching sketch and you decide 
to sell it, you should sell them together.  

But I think if you have the cel and you are 
willing to part with the sketch and someone 
wants it...by all means sell it.  What I think is 
taboo is if you sell *both* the sketch and the 
cel *separately*.  And if you decide to offer the 
sketch to someone and later on down the line 
you have an emergency and *really* need to 
sell the cel as well, I think you should offer it to 
the person you sold the sketch to.  I know, I 
know...you may not remember who you sold it 
to.   


Anyway, that's my newbie two cents.  They 
should be sold together if you are selling both.      
I'm not sure if I'm being clear but I think if you 
are a cel person (you like the cel but don't care 
for the sketch) and you want to sell only the 
sketch, ok.  And if you are a sketch person  
(and don't care for the cel), go ahead and sell 
the cel separated.  **BUT** if you wish to get 
rid of both, sell them as a set.

Damn...I'm long winded. 

- joyka