How much is my PC worth? Well that is a very difficult question for anyone to give an answer to as you haven't given much information away. For instance you would have to give some indication of how much you paid for it and how old it is.
My opinion is that you won't get anywhere near what you paid or what you expect to get because no matter how good the spec. (and yes it is a decent one) it is now secondhand. No doubt this isn't what you expected to hear.