I'm a novice in this regard, but I will say that 
it has often been mysterious to me just how to 
deal with "Offers Taken" situations.  When 
there's no range given, then I assume any offer 
is at least entertained, even if it is rejected.  
But even though I've followed eBay and a number 
of reputable dealers, I know that the price of a 
cel depends on lots of variables, most important 
being what the owner paid.  

There's a creepy sequence in CCS where Sakura is 
tied up in vines crucifixion-style as part of her 
Final Judgment.  There's an unsettling sequence 
in which the camera pans verrrry slowly up her 
bound body.  The studio made several big pan cels 
of this image and cross-cut among them, so that 
the broadcast image "quivers" in a weird way.

I saw two go for sale in 2001, one for $75 and 
one for $125.  A third showed up on eBay soon 
after.  I bid $125, and that bid was passed 
minutes later.  For some reason, I really wanted 
that cel, so I was tempted to go over my usual 
limit of $150 ... but by the time I overcame my 
scruples the bids had gone over $300.  Final 
price: about $850.

Now if I had offered the successful bidder $90 
for his cel, he'd have laughed and sent me a 
sarcastic e-mail calling me a naive fool.  Yet 
that is exactly what I did pay for the next cel 
from this sequence that came on the market.

So it seems as there ought to be some basic rules 
for "Taking Offers" situations.  First, if price 
is a consideration, there should be an 
honest "starting" price.  Of course you can't 
automatically expect to get it for that if 
someone else offers more.  But if no one else 
wants it, the seller should be willing to honor 
that price.

(Some sellers have given starting prices, then 
rejected my offer at that price, then sold the 
cel later on eBay for that price or less.) 

If there's no starting price, then offers should 
at least be met with a counteroffer.  "No, I 
can't take $90, but I would take $900."  "OK, how 
about $250?" "No, no, I paid more for it than 
that.  How about $750?"  "$500 and my Ryoko cel 
in trade?  Deal?"  "Deal."