What are the use cases for the second hand market such that prices can rise exponentially? Just hacking and perhaps at home testing for open source projects that need to run on the widest array of ISAs? Or is it just a scarcity thing?

It's primarily scarcity. Unless you're in the know and are a special kind of computer geek (see Cameron Kaiser of Floodgap Gopher fame) - most folks aren't really going to know about Raptor's offerings. Very few folks are buying them new, thus very few make it on to the second-hand market.

I really want IBM's Power ISA to have more mindshare, as I think it's a cleaner design and more straightforward than the likes of ARM or RISC-V. Unfortunately, IBM is more concerned with wringing out every last dollar from their contracts instead of pushing the ISA out as much as they can. OpenPOWER was supposed to be this initiative, but it has seemingly fizzled out to a large extent, such that NVidia and Google no longer are Platinum members and hardly anyone talks about it anymore.