Aluminum is pretty hard to work with, but a body shop is probably your best bet. Or, you can try a block of wood and a hammer, if you're brave. But like jsw said, take everything you can out of it (right down to the motherboard, if you can manage) before anybody starts pounding on there--bad business banging electronics around. The hard drive would be almost guaranteed to break, and who knows what else might pop loose or crack under the strain (those G5 heatsinks are HEAVY after all).
Sounds suspiciously like the seller had already damaged it and prefered to try and make it look like the shipping company did it than sell it as-is. It'd certainly explain why he/she wouldn't want to insure it.