Buying New, Used, or Fixing Up
Buying a New Mac, iPod, iPad, or iPhone
Mac Buyer's Guide: Know When to Buy Your Mac, iPod or iPhone
http://buyersguide.macrumors.com/
Is it a good time to buy or replace your Mac? This site shows when it was last updated and uses past trends predict when Apple will release a new model. Also covers the iPad, and iPhone.
Buying Used
Apple sells used equipment directly, and while the prices may be higher than other dealers, you get a full Apple warranty. To shop for used equipment, look for “Refurbished and Clearance” in the footer of apple.com.
If Apple’s selection is too limited, Best Buy also sells used Apple equipment.
But one of the best sources for good deals, especially on older equipment, is Mac of All Trades.
Fix or Buy Replacement Parts
IFixit
http://www.ifixit.com/Guide
Repair manuals and detailed information about the internals of your computers and gadgets