Used Beretta 92/M9 in good condition are about $350 to $450. 92A1/M9A1 are $400 to $500. Trade value for an M9A1 in halfway decent condition is $450, at least. Unless it was beat to shit I'd have told that dude to eat a bag of dicks if he tried to say it was worth only $300. Hell, the only time you see 92's for sale at $300 is if they're well-worn police trade-in pistols where the guns are battered and the finish is halfway gone.
That said, the 92 is a $700 gun sold by the factory for $600, while the public sees it as a $500 gun. They don't hold value unless you get a Brigadier or specialty model. Which is odd, considering the 92 is equal to the P226, yet costs a could hundred dollars less.