Typically I set a price that I think is "fair" for a machine then hold firm on my price when selling. When buying I pay what I think is a "fair" price regardless of what seller was originally asking. I can't begin to tell you how many times I paid more than asking price for project machines advertised on Craig's List or Mr Pinball. Doing that has opened more doors for future deals than you can begin to imagine. Both of my warehouse deals came from being more than fair with seller on original machines I bought from them.
My wife thinks I am crazy but it works for me.