Well I have been on the other end of this a few 
times when selling a few relatively cheap cels 
(ie under $50). 

I think it all depends on what you are paying for 
it and how much it was ramped up because of the 
non payer. If it is a low price (say under $50 or 
so, depending on what you consider low) I would 
suggest you either simply accept it and pay up or 
negotiate to split the difference between your 
last bid before the new commer and the final bid. 
That is certainly how I have handled it. The 
problem is that this new non paying bidder may 
hade discouraged others from bidding over your 
initial (before the new commer arrived) price, 
and hence the seller may also have gotten a 
better price if the new winning bidder never came 
(if this makes sense). Maybe he wouldn't have but 
you never know. 

So unless this has changed the price by say over 
20% I would suggest that you negotiate to split 
the difference. Otherwise simply say no and wait 
for it to be relisted. From a sellers perspective 
if the difference isn't great they will probably 
prefer to sell the cel rather than go through the 
trouble of relisting it wand then waiting to see 
how it goes again....

I have had both experiences and been happy with 
both results. I guess it all depends on if you 
feel like you have been fleeced by the seller or 
not and if you feel you are then why deal with 
them further?