resulting page will show you that a wi-fi only iPad 2, 16GB (white) with a grade of "like new" will net you $320 from Amazon.com, or $288 if "good," or $236.50 if "acceptable."
That same device would only net $185 / $205 from Gazelle.com (Good / Flawless), one of the most popular electronics trade-in sites.
BuyBackMac offers $220 / $250 (Normal Wear / Flawless) or NextWorth will offer $242 / $254 (Good / Like New, but in both cases including the original box; $10 less otherwise). Other than Amazon.com,, eBay has decent pricing, at $290 for a similar trade-in.
Why the big difference in offers? Amazon.com can, of course, resell the items directly on its own website, which eBay can do as well. But Amazon.com has a bigger advantage: it doesn't pay the price to the seller in cash, but instead in Amazon.com gift card credit.
That's no big deal for those, like us, who buy a lot on Amazon.com, but it also means that Amazon.com is assured to get its payment back in the form of later purchases.
We'll also add that Amazon.com takes its sweet time at "judging" a product that you sell them. Once they receive a trade-in it takes so long you might even forget you traded something in (we did).