Time to Trade Up?

The Boston Herald is reporting that electronics retailer Tweeter Home Entertainment is launching a program where customers can trade in their old iPods for credit towards a new one, just like car dealers do with automobiles. The iPod's value is calculated by answering a series of questions posed by a sales associate, who then uses a Tweeter price guide (which the company compared to Kelley Blue Book) to figure out how credit the company will give you. The canton, Massachusetts based company is testing the program, a first for the iPod in the United States, in their Boston area stores first before taking it nationwide.
On the surface, this idea is pretty cool concept and I'm curious as to why other retailers haven't tried it, given the iPod's popularity and tendency for frequent upgrades. I do wonder though if you'll be getting more value for your iPod from Tweeter than you would from, say, eBay.