How do I determine the value of a car?

Check various model related websites. That'll give you a good idea. I'm just trying to sell a car now & it being the third iteration of what I've built I know pretty well what to expect. I'm not looking to make a ton of money, just what my experience shows is reasonable based on the car, what I added & my time.
Now will I get what most guys ask ? Probably not. But I'm not expecting to. You need to find a reasonable seller, not low-ball him and be realistic about the car. That way you both go away happy after a non-confrontational (?) sale.