Hi and be welcome!
I've heard about these machines before. They are indeed used by developers (and some universities). Despite that, I cannot tell if the device offered is the real GameCube development kit (actually, if it is, it should come with PC software to develop games with). What you see on eBay may well be a skinned GameCube. I honestly don't know.
BTW I moved this to the console forum, that's where it fits in the best