I suggest you use Windows 2000 Professional or Windows XP (either Home or Professional will do). I do not recommend using Windows Vista at this time. Windows 2000 does not have software activation so if you just want to run Windows in a VM under Parallels that is what I would recommend, if you can still find it.
If you want to install Windows via Boot Camp and then use that installation under Parallels, then you should use Windows XP. Windows 2000 or Windows Vista won't work in this case.
The problem of buying from eBay is that you may not know whether the software had been activated previously (in the case of Windows XP or Windows Vista). In such a case you may have problems activating your copy after you install it.
Last edited: May 21, 2007