of the 4 macs I own, only one was purchased new (Apple offered me $1000 off if I turned in my PB 5300), two are from ebay and one was a factory refurbished box. If you go used, have a mac savvy friend help you understand the buzzwords, (an ebay savvy friend would be a help as well)
I confess I design my macs from the screen back, first I buy the monitors I want, then the mac I can afford to drive them. my desktop (bought refurbished in 1996) drives dual 20" monitors bought used, one color, one grey scale; My laptop 14 " screen can drive dual external monitors as well, for me that is what I need in a Mac, screen space. I tried to remain child-like, all I acheived was childish.