I just bought a Canon ip4000 printer on ebay, but first, I did my homework. 
I googled printer reviews online, reading what others said about the printers' performance AND the price of the cartridges. 
Then I googled for best prices out on the internet including the larger national computer stores.  Then after narrowing it down to:
1)  What I mainly planned to use the printer for,
2) Cost of printers
3) Cost of cartridges,
4) Reviewers' ratings, I looked for that item on ebay.
 I clicked on Add to favorite searches, and when I received Notice emails from ebay and
1) the price was right and
2) and the seller had a good feedback
3)  the shipping cost was reasonable
4) and the manner of paying was agreeable, I put in my bid.