I find it a little strange when selling cars too

parts is a little different cos pricing them up can be awkward,but generally you should have a figure in your head your happy with

if your selling a car you usually have 3 figures in mind;

1) what the car is worth to you,i.e. what you'd like to get for it.
2) the lowest offer your happy to take for it
3) the lowest you can AFFORD to go on it.