They're reputable. Haven't seen a lot of discussion of people selling used Macs to them, but when it comes to product take-apart guides and parts, they're some of the top people in the business. I wouldn't have any hesitation doing business with them.
That said, you can generally do better by selling directly on your own, as it cuts out the middle man. Of course, there are hassles and risks that go along with it, so it's always a matter of preference.