The thing about Car Max is that they base their offer off of what their market for the vehicle looks like. They have over 125 dealerships nationwide, and they sell hundreds of thousands of vehicles a year, both at retail and at auction (they actually operate their own auctions now, too). So they have a very accurate barometer of what they'll get for teh vehicle, and there are very standard procedures for profit margins and what not. In the end, they don't care what the blue book says. And this is true either way: They offered us $1800 more than the blue book trade-in value for my wife's first Rogue, because it was a hot vehicle at the time and they were flying out of their dealerships, especially with lower mileage. We then took that "offer" to the dealership where we purchased her Pathfinder, and negotiated an even higher trade-in value for the Rogue there, despite the fact that their first offer came in about $2,000 under Car Max's offer (regular dealerships almost always offer you less than the low blue book on their first shot and say there are "refurbishing charges" and what not, even though Kelly Blue Book already factors that in).
They know what they'll get for it, so it's a pretty simple process to make you an offer once they inspect the car. And it really could go either way: If they have a bunch of people looking for Xterras and they're flying off the lots, they may offer you more than you think for it. If it's going straight to auction (too many miles, needs major work, etc), or if they have Xterras sitting on their lots around the country that aren't selling well, they may offer you considerably less than you were expecting. But it never hurts to check - the appraisal is free and there's no-pressure, and they even honor their offer for a week after they've made it so you can think it over.
