The K750 (black/Win version) is still available but has severely gone down in quality. I have a personal one that's about 6 years old and they seem to have gone

The K750 (black/Win version) is still available but has severely gone down in quality. I have a personal one that's about 6 years old and they seem to have gone through an undisclosed iteration (no new model number, although you notice tiny differences) - I recently tried to buy four new ones for another deployment - they were ALL faulty, slow or creaky and had to be RMA'd. I was stupefied. So you're probably lucky that you got an "old" model from eBay anyway ;-)