Have you shot the gun yet? Does it shoot were you are pointing and eeject the spent shells ok? A used gun with the rails looking a little worn seems to be logical to me. The nick...... I would really have to see to know weather it should be a concern. I have never pulled the trigger and looked at the end of the barrel before......I don't think I am going to now either. There is a slight gap at the top of the barrel between the slide and the barrel.
Good luck the CW9 is a very nice gun!