i always ask whats wrong with the stuff and gauge how much i think it is worth before i go out, i get that much money and put it my pocket, i also get a little extra incase it is in better condition than i though and put that in my wallet. i usually have a total amount equal to the asking price but i do not make any offers until i see the item in person because if you tell them what you are offering over the phone they are more likely to turn you down as opposed to in person because they see the cash in front of them and you can pick out things that you think should lower the price like balding tires, scratches and other things.