I also don't like sellers doing that, but if you
put yourself in the shoes of a seller who only
wants to get money from his/her stuff you cannot
do anything about it.

I have thought about a similar example. Imagine
you own an entire cel sequence, lets say, for
example from A1 to A10 with one original
background and the 10 respective sketches. Now
imagine that you want to sell it..... you could
always try to sell the entire lot but definately
you would get much more money if you split the
cels and sell them separately.... and even more if
you make 9 lasercopy backgrounds for the other 9
cels of the sequence.....

If you think about that situation you can think...
"I don't mind because each cel is different", but
you have to consider that it's just because it's
not a common practice to collect entire sequences
and instead it's very common to collect cels with
their respective sketch....

With that said, i repeat, i'm against the practice
of separating cel and sketch.