I have a Heritage {same as a sidekick but camo stock} and its a tack driver.I have the bushnell trophy on mine and shoot 295gr Powerbelts with two T7 pellets,nice results
The MU plug is dream come true,it sounds like that guy may have the Hubbards ram rod which is a nice addition compared to the factory one which is weaker.I would think its a fair price if you could get him to 300.
Busta might be better at pricing it then me.
Either way if you get it you won't be unhappy.They are great guns and can't wait till Big green comes out with the newer version.