Yes, they are. Small Dog up in Vermont use to see parts but I think they ended that part of their operation for some reason. Maybe part of their license to sell new and refurbished Macs.
People use them to replace parts, not to buy a Mac. During the phase with Tiger there were only OEM copies of Mac OS, there was no retail copy to buy.
Only Intel Macs run SL. SL will install on Tiger or Leopard or you can buy the full box set. Amazon is cheaper, $25 and no shipping or tax.
If I was looking to buy a Mac Pro I would of course first look at Apple Store Specials. Not much variety, and none are low end.
http://store.apple.com/us/browse/home/specialdeals/mac/mac_pro
But the $2199 - 2399, Mac Pro 2008 8-core, or 4-core 2009 2.66GHz is a nice system.