for low priced general parts and things, i usually go new price /2 as a fair second hand value. half again if it's in shoddy condition
for something like a car, well... a car is only worth so much. i can spent $20k on my ke25 and still have it as just a 32 yearold corolla. i would tend to look at other similar things about the same condition and decide off that. some people tend to overinflate the price of their car, some go for a song. there are things such as the Unique Cars Vaule Guide to help with this
when other people want bits that i have, i'd prefer to swap something that they have, rather than sell it. money is only so useful, and so short-lived. rare and/or useful bits are much more satisfying