Here, G4's you can barely give away, except for the MDD dual 1.42 (now unobtainable here) and the now noticeably appreciating dual 1.2.
G5's are hit and miss, low spec are barely sellable, anything with water cooling is still well over $100, for now. Mid range G5's are a lottery, but not fast sellers.
(Mac Pro 1.1's have come down massively in the last year too)
They most certainly are! Remove 4 screws from the base of each water block, lift the cap, remove the O-ring from the plastic channel they sit in and replace them. Finding the right size can be tricky, but not impossible. I've had companies custom press new O-rings to the correct dimensions for automotive use that would probably work fine in a Power Mac G5.
I wouldn't be surprised if you could even seal the blocks with automotive gasket sealer rated for cooling system use. Wouldn't be as simple of course, but in a bind and being a relatively low pressure system, it would probably do the job.