Buying from Gamestop is convenient since you'll have a place to quickly and easily return it if it's broken, especially if they're used. Looks like Gamestop is selling refurbished/used models for $50. If you're wanting to buy used I'd say aim for a Gamestop so you have that added peace of mind, even though you'll pay more for a used model from them vs Ebay. They're also currently selling a new Kinect for Xbox One for $57 (http://www.gamestop.com/xbox-one/accessories/xbox-one-kinect-with-dance-central-spotlight/116957
) which seems like a strong deal considering Microsoft's MSRP of $99.99. I can't find a better price online at the moment.
Seems like the best deal right now for a new unit, and again, you can return it if anything goes wrong. Here's some options from Ebay for comparison: (https://www.ebay.com/p/Microsoft-6L6-00001-Kinect-Sensor-for-Xbox-One-Black/710143192