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Indeed, Macs have a great resale value on eBay but you know why - Mac users hold on to their Macs forever. I have a seven year old G4/400 - and I have no desire to sell it. Macs hold their resale value because people hold on and keep using them for a long time. So, while yes you can eBay a Mac every 2-3 years and get a new one - more likely you'll keep using it for a long, long time. Or, just add to your collection.
I had a G3/600 iBook - I got a newer iBook G4 and had intended to sell the iBook G3... but then I thought I could dedicate it to iTunes and keep it near my stereo and pretty soon it had an USB->optical audio gizmo from M-Audio and an infrared remote and I kept using it exclusively for iTunes. Eventually it's hard drive died (as all hard drives do) and not wanting to replace it, I sold it for parts on eBay (still brought in $100) and got an AppleTV to take its place.
The various anti-Apple hordes of letters are all over the place. The guy who switched to PCs when XP came out - apparently hasn't heard of MacOS X because XP came out in 2001 right around OS X 10.0 - so he's complaining about instability in MacOS 9. Nice.
Simply put, PCs are a dime a dozen - why would anyone buy a used one. They're indistinct collections of off-the-shelf parts. This does mean you can, build-your-own machine or do a lot of upgrades and great for you if that's what you want.
But each Mac is unique and remain usable for many, many years - and this does enhance their value - which is why the used Mac market exists. But the Mac's high resale value doesn't mean you'll be reselling a Mac every 2 years - it means you'll hold onto it for 5 or 6.