I would say the resell value of Apple hardware the comes down to a number of things.
1: Cost of information.
While information online may be free, the time spent finding the information is not. So a buyer may have decided to spend 500 "utility", where 1 "utility" equals one dollar or one minute. This is what we do everytime we use a taxi or hire a professional, trading money for sparetime. Or opposite when we work, trading sparetime fore money.
Apple have an extremly easy lineup compared to most companys and especially compared to the industry as a whole. So the consumer may spend $500 on a used Mac (assumed cost of information zero) or 100 minutes and $400 in finding a proper used PC. All numbers used is of course only for illustration.
2: "Speculation".
A buyer of used Apple hardware is prepared to pay a premium compared to a buyer of PC hardware because she knows that she will be able to charge a premium also when she sells the hardware. No one but Apple gains on this though.
3: Aesthetics
Most of the Apple lineup from 1999 and onwards have been looking good. Men crave for good-looking things, from women to clothes to cars to computers. And we're ready to pay a premium for good looks. So even if the hardware is dated the looks remain and add value. This can be seen with the titanium/aluminium powerbook where the Al PB command a premium which can't be fully explained by its newer technology alone.
4: Lack of lowend-offering
For someone wanting a Mac laptop, the least expensive start at $1100 while the least expensive pc laptop start at the $500 mark. If you can't afford $1100 you're forced to buy a used laptop, increasing demand and raising prices in the sub-$1000 market.
The Amazing Resale Value of Your Mac