You are totally right about the problems of 
pricing.  One problem that I come across as a 
buyer is the seller's unwillingness to bargain.  
When someone posts "make me an offer", that puts 
the price of a cel somewhere in a nebulous fairy 
land the vague location of which resides 
somewhere in the mind of the seller.  This puts 
the burden of guessing on the buyer.  It may be 
easy for someone who shops for a specific series 
or character a lot to put a reasonable value on 
a cel, but for someone who doesn't it can be 
difficult and frustrating-and as Drac posted a 
lot of people don't have time for that.  
Especially when potential sellers are rude, 
unresponsive, or break off negotiations in the 
middle (presumably to sell to someone else). 

If the seller doesn't want to put an exact price 
on a cel, they should at least post a ballpark 
or 'starting bid' on the cel.  Most private 
sales with no set price are private actions 
anyway.

Gee, I didn't mean to sound so bitter!  Can you 
tell I got burned recently? :P