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After just a year or two of use, a Windows machine gets so gummed up with spyware, viruses and other nasty stuff that it seems malicious to ask anybody for money for the thing.
That's hardly the reason for the poor resale value of a PC. In fact, anyone who knows anything about reselling machines knows they should wipe the system (and hopefully reinstall the OS) before selling it; the alternative is sending a machine full of your private data to the new buyer. (If Mac owners don't do this, they're even less tech-savvy than I'm giving them credit for being.)
In general, PCs have poor resale value for two reasons:
It's worth noting that many PC enthusiasts actually do something similar to what you're doing now; they buy the latest and greatest parts for their machines every few months, then sell them on eBay for around 75% of the original price to fund their next acquisition. Like buying a Mac, this still reflects a certain diminishing return -- but it's a price they pay for their lifestyle choice.