You definitely should end up with a conclusion in 
which you don't get "taken".  You should at least 
break even (where cancelling the whole deal is 
one way to break even), and ideally you should 
come out on top because the dealer feels bad and 
you have not cut off an opporunity to let them 
offer you some recompense.

But, as one other person noted, this happens more 
often then you might think.

Most cel dealers are honest and good people, but 
many of them are VERY disorganized.  The fellow 
who is probably my favorite dealer is about the 
most disorganized person I know -- sells me cels 
that he's already sold to someone else;  brings 
cels that he has not yet priced to conventions 
(and as a result I get a REALLY good deal when he 
thinks up a price on the spot);  leaves on 
vacation without telling his assistant about my 
deal that we're halfway through; etc.  A really 
good guy, but a total flake.  (I'll bet that some 
of you reading this know exactly which "C." I am 
talking about....).  I stick with him because 
he's a good guy and I win overall, and he gets a 
lot of amazing cels from Japan.

I think that this might be part of the artistic, 
right-brained nature that attracts people into 
becoming dealers of art such as cels.

Anyway, keep yourself financially whole whatever 
happens, but the evidence suggests that you 
should assume that the dealer is a flake (as 
opposed to evil) until there's some reason to 
think otherwise, and I'd suggest graciously 
pressuring the dealer for some compensation for 
your loss that works out as a win-win for 
everyone (but especially for you).