Keep or toss?

[energiesource]

When it comes down to the taking everything apart, how do you decide what to keep and what to throw away, i know this seems like a stupid question but ive just been tearing everything apart and i dont know what to keep or throw away, what i can get brand new and what i need to keep kinda thing. Fenders are straight forward if they arent rusted i can use them, but other things like relays, heater cores, that stuff. Can anyone help me?