My recommendation is to put them on Craigslist for free. I can't think of a time when I wasn't able to at least give away old computers / computer parts. Someone might be willing to pay the cost of shipping a free Mini but the cost of packaging and shipping a G5 tends to make them local exchanges. As someone who had to ship a G5 just a few months ago it was a PITA...and that was using the same packaging it came in.
What are the configurations? If you have any later model G5's, especially the quad, you should be able to unload them for free fairly easily.