The only way to do this in person, or with lots of REALLY GOOD pictures. Even then if you ask 5 people, you will get 5 answers. Problem is that price and value vary depending on what you want from the bike. The only good way to go is to watch bikes sell for a while, determine what you really want from the bike, and only then can you put your own value on one that's for sale. If you aren't sure what it's worth, you haven't been looking long enough to buy.
Just my opinion