Wavelab 6 was the last version to officially support Windows 2000 and Windows XP and hardware that requires them, which some facilities and people still rely on.
It also has a completely different workflow. I think pretty much everyone will agree that Wavelab Pro 9.5’s workflow is far faster, but some people are weird and might prefer to do things the old, clunky way. 
You probably wont find a buyer right away (this is a very niche product), but I think you should be able to get around $100~$200 for your Wavelab 6 license.