I've had very good experiences selling used Macs on eBay.
Write a good auction title with good keywords so bidders will find the auction, as well as a detailed description with good, well-lit photos of the system from all angles. Guarantee that the system will not be DOA and emphasize that there's nothing wrong with it (and if there is anything wrong with it, be forthright about it). Take screen grabs of your About This Mac… screen, the Overview pane of System Information, and any other things you think bidders will be interested in. Be sure to blot out your system serial number from your screenshots.
Shipping is the real headache for large desktop Macs, but if you have your original Mac Pro box, then there's no problem; you can just repack it. Always insure the shipment on such expensive items.
If you're in the continental United States, I recommend restricting your auction to bidders only within the continental US. Overseas shipping on desktops is begging for trouble. (I tried it once with a G4 iMac. Big mistake. On the other hand, I once had an excellent sale of my iBook G3 to a buyer in Germany.)
Ground shipping cost is usually reasonable within the United States and I almost always offer free shipping as a way to entice bidders.
Don't forget to erase your Mac and set it up as new before selling it.
I'm working on an eBay listing as we speak—with great reluctance, my 2010 Mac Pro will be going up for auction in the near future. It's time to part with it. 😢
Good luck and have fun!