The very first offer that is made should incite a slight negative reaction from you. Decide what feels most comfortable for you and then perfect it. This may be a slight raising of the eyebrows, a crinkled nose, a little gasp, or a slight frown. Often this will be enough to have your opponent backtrack or make a counteroffer. If not you may want to follow up with a simple statement that while the item is certainly worth the amount being asked it simply is not something you can offer.
Alternatively, if you are selling an item or service you could respond by saying that while you would like to see them leave with the item you couldn’t possibly agree to something that wouldn’t be fair for both parties.