Enter your gadget model (e.g. “iphone 3gs 16gb”), tell ’em what condition it’s in and whether you have the original cables (e.g. “good, makes calls, no to A/C & cables”), and they’ll tell you what they’re gonna pay you ($190 in the previous case). You say “Great!”, they send you a box, you send them the gadget, they send you a check.
If you’re a non-profit organization, Gazelle will give you a page through which donors can send their gadgets, and Gazelle will give the money to the non-profit.