I would have bought it for £20 when we were doing ours. Which would have been a mistake as BMW bought us a new one out of the goodness of their hearts. But I wasn't to know that they would be so kind. If you are not in a rush and your car has not been serviced by BMW it definitely has value.
the roof motor swap was easy and quick (less than 3 hrs next time) and a failed roof is not such a big deal.
I didn't buy second hand but if I needed a roof motor for an older Z I would probably give it a go.