Offer periods are ... difficult.

The buyer runs the risk of being laughed at or 
worse for offering too little, spending too much 
because he/she didn't know the market value, OR 
being left totally in limbo with no response.  
Most of us are better than to treat potential 
buyers this way, but it happens -- too often.

On the other hand, from the seller's standpoint, 
maybe you really don't know what a shot might be 
worth to others at this time.  In that case, 
offers make a lot of sense.  Especially given 
eBay's current fee structure.

What I like better has been called an offer 
period but is really more accurately a private 
auction page.  Limited time, here's the scope of 
what I'm willing to part with and the minimum 
I'll take for it, but higher offers will be 
entertained until x date.  That way the buyer 
knows whether there's any point in pursuing the 
shot, the seller has the possibility of making 
more than his minimum, and we don't have prices 
on certain cels spiraling ever higher to cover 
middle man fees.  (We also don't have the limited 
protections ebay offers, but hey).

As far as etiquette goes, it's just common 
courtesy to respond to all offers, do it clearly 
and politely, and understand that not everyone 
values things the same way you do.  

-- I suppose if we all really understood that 
last part, we wouldn't have wars, would we? :-)