Speaking for myself, for the most part I only 
have myself to blame if I end up paying too much 
on certain cels.  A dealer can charge whatever 
they want for a cel, but it's the customer who 
agrees to the price who decides whether or not 
they got a good deal.

I can say that I didn't know exactly what was a 
good price for a certain cel from a particular 
anime earlier on.  And because of that, I 
sometimes found myself buying cels that I ended 
up regretting (either because of the price I 
paid or because of the cels I ended up with).  
Only experience (and regular vigilance over the 
cel market) can teach you how to appraise the 
worth of a cel.  Although cel prices fluctuate 
greatly, you still do get a better feel for them 
over time.

I will concede, however, that certain cel 
dealers may charge prices that are higher than 
typical market value (and sometimes higher than 
reasonable), because either they know some 
collectors will be desperate enough to pay those 
prices.  It's not a practice I appreciate, but 
it's within the dealer's right (and if a 
customer is desperate enough to pay that much 
money, then I have to say that the customer can 
only blame themself).

What I do not condone in any way is if the 
dealer misinforms the customer or misrepresents 
the cel's condition, quality, and/or size.  A 
few dealers (mostly auction sellers) can 
manipulate a cel's scan to make it appear 
larger, more pristine, and/or better than it 
really is in some other way.  Or the dealer can 
omit or flat-out lie about a cel in their 
description.  The best dealers will be honest 
about any potential problems with a cel before-
hand.  Any act of misleading the customer should 
be intolerable and it should prompt you to avoid 
doing any further business with that dealer.

Another possible problem with a dealer is that 
the cel arrives to the customer in a bad 
condition (or sometimes even the wrong cel is 
sent).  If the dealer makes a mistake (that is 
clearly theirs), it is good professional 
courtesy for the dealer to try to correct it 
(exchange the cel, provide a refund, etc.).  A 
dealer that doesn't take responsiblity for their 
foul-ups should also be a dealer you should 
avoid in the future.