I don't agree with what the seller is doing...   
But they are his cels and his sketches, and he is 
allowed to sell them together or seperately for 
whatever prices he pleases.  (Unfortunately)

I had that happen a while ago also... I bought a 
rather expensive cel from a private collector and 
when I received it, no sketch was included.  Then 
a few weeks later I went back to his site looking 
for updates and noticed that he now had the 
matching sketch of my cel for sale at almost the 
same price!  I was more than slightly peeved, but 
it was my fault for not asking if a sketch was 
included with the cel or not.