man I don't like a low ball bidder... "see you have $150 on it but I'll give you $30..."

will explain how it works...

you don't want to pay the price? first ask the guy if is willing to take less for it. if he says he will consider it then he just gave you permmision to offer him whatever you had in mind. he can either accept or decline... but it shows he is willing to work with you.

outside of working like that if you feel the need to blurt out a number then keep it respectable.