It depends on if you trust the seller. If you know him, and trust him, I wouldn't hesitate, go ahead. Is he a stranger? If yes, you should negotiate. Have him demonstrate the flight range and see how far it can go before it disconnects and RTH. This is a good way to negotiate him down since the advertised range is 3 miles, and it's rare that anyone can get 3 miles range from a P4. Have him show you the number of charges on each battery (it shows in the Go app) so you can determine how many flights he has made. Generally speaking used Phantoms are sold for 50 to 60% of their new price if they are in good working order. They don't seem to hold value much. The really good buys I've seen are the P3P craft, usually 50% of new, however the condition is important on those. You'll need to inspect for cracks around the motors because some of the shells were weak. Both of my P3P haven't cracked, no problem on my two P3Ps. I fly my P4 more now, but I had to pair an Inspire controller (GL658A) to it so I could get good range, comparable to my P3Ps at 2+ miles. The stock P4 controller (GL300C) is weak, the weakest Lilghtbridge controller DJI has made to date. I was lucky to get 1 mile range with the stock P4 controller.
