This is just my opinion but if the gun looks to be in good condition it is absolutely worth $400 with 350 rounds of ammo. I wouldnt hesitate to buy it. I have owned alot of guns and I can count on one hand how many I got to fire first before I bought it. The XD9 is a very good firearm and to get that plus 350 rounds for around $100 less then a new one I would jump on it myself. They run around $499.99 new but add to that around $60.00 - $100.00 worth of ammo depending on what it is.
"Perfection is the enemy of good enough"