Looking at the Buy Now price is a good start, but unless you're in a hurry to sell, follow a few auctions on Gunbroker and see what they actually sell for.
Remember that there's a marketplace area on this very forum — give some of your buddies here a crack at it.
And there's probably a state or regional gun forum that covers your area and has a buy/sell/trade section. If you sell to someone within the state, there's no need to get an FFL involved in the sale/transfer. You might get lucky and find someone with a Raptor for trade who would give you a fair shake on your PX4.
"An armed society is a polite society." — Robert A. Heinlein
"After a shooting spree, they always want to take the guns away from the people who didn't do it. I sure as hell wouldn't want to live in a society where the only people allowed guns are the police and the military."
— William S. Burroughs