I've only had one good experience with making 
offers, for a cel from a wishlist scene. For the 
most part, I tend to avoid them. I'm not very 
comfortable with it, because though I'm familiar 
with what general market prices are reasonable, 
sellers are often quite a bit more optimistic 
about what they want to get for their cel. ^_~

You might just sell it from your page with a set 
price. You might get a little less for it than 
what someone might offer or people might compete 
for-- but by the same token, you won't frighten 
people away because they don't know what you're 
expecting to get from the sale. 

Or you might put it on Ebay or do an Animanga 
auction. Don't make the opening bid lower than 
what you're willing to accept-- it leaves a bad 
taste behind when some people end an auction 
early because the bidding isn't going the way 
they'd like it to, or if they back out completely 
after the auction concludes.