You can probably do pretty well on those locally! I would Craigslist them for $220 OBO and see what offers you get. Just list it as if you have one system to sell, it is nobodies business that you have many. Offer a mouse, keyboard, and (if you have extra) an LCD monitor with it.

Those PC's on ebay are fetching less than $100 when fees and shipping are considered....you can do MUCH better locally.