Not only is the practice legitimate, but it is also rather common. Before you purchase a computer, whether it's new or old, you should read over the warranty very carefully.
Agreements are often null and void once a computer changes hands.
If you're thinking about buying a used computer, ask to see the warranty. Your case isn't hopeless. You likely can get your computer fixed, though you may have to pay for it. If the manufacturer flat-out refuses to look at your PC for a fee, try taking it to a computer repair shop. Using the Internet to meet someone online is a very convenient, effective and enjoyable experience.