Used system, ebay is fine if you found some there. I'm sure you also have local sites like Craigslist in your area also to look at. For a used system there is no single cheapest source since what is available and how much it sells for will vary a lot. Someone could be selling a very good system cheap for a quick sale when you look, or it may be gone already and you paid 50 more for the same thing without ever knowing about it.