A general note. Most of the values given on sites like this are RETAIL values, as are the values given in the gun price guides. In other words, those are the approximate prices on a private sale or what one would expect to see on a dealer's price tag.
Most dealers will try to buy as low as possible (surprise!) and certainly will not pay more for a used gun than they can pay for a new one wholesale.
As for trade-in price, that depends on what the customer is buying. A dealer will not give much in trade for a $500 gun on another $500 gun, but if you trade a $500 gun on a $5000 gun, the trade-in value might well be $500, essentially a wash for the dealer, but he makes his money on the $5000 item.
Jim