I haven't had many issues with used Macs from eBay. In fact, I usually buy ones that are broken with some minor issue that is easy to fix with parts as my daily use laptop, but I have on occasion bought one that has me sending it off to repair the logic board. I will use them for a while until I either decide that I feel like upgrading to something newer or someone in my family decides that they want my current MacBook Pro. It's more like a fun little hobby for me that stems from my teenage years of building and repairing PC's for my summer job in the mid-to-late 90's.
Getting back on track, there are some good deals to be had if you want a good used one. Some people even have a little bit of AppleCare left, if you feel that a fairly current Apple Store refurb is either too expensive or too new for the child.
