I don't get it either for underwater still work. The camera crops to the center of the frame (either DX on a FX cameral or 1.3x crop on the new D7100) and I can always do that myself. However, what I can't do underwater is compose as well as I can on my 24" screen at home. Plus, it's a pain to move the focus rectangle around to get a focus on the eye and still get a nice composition. It seems much easier to leave the focus rectangle in the center and fire away, using all those extra pixels to compose on land.
It is a good deal if it works. Ikelite has very good service so whatever is wrong with the housing can be easily fixed. Check for scratches on the dome but, unless, they are serious it is not a big problem.
The Tam 17/50 lense is not a normal choice for underwater so I would find out from ikelite what lenses will fit in the port setup.
Simply have the guy show you how to set it up (meaning your hands on the rig) and take an above water shot, then submirse it in fresh water for a spell. A decent shot and no leaks is a go.
I would not be suprised by a good deal. Often used rigs sell for less than many think they are worth.