Since the time of Ford's Model T, there's been a steady market in used auto parts. Early computers generated some parts trading too, but that hasn't been the case since Intel's Model 8086 microprocessor transformed the desktop. Because used components of the past decade's micro- and minicomputers are so hard to find (and because desktop-computer performance has improved so dramatically), businesses tend to trash computers when hard drives grind to a halt or power supplies burn out. That's really too bad. Granted, the older machines, if repaired, probably wouldn't run Windows at speed, but they'd provide someone with a reliable word processor.

Now they can do so. PartsPort, in Petersburg, Va., has published a listing of hard-to-find repair parts for IBM-type computers and peripherals that it carries in inventory. Obscure but essential items include an ITT transformer ($1), a Xerox laser cone roll ($38), and an IBM clutch ($165). For a free brochure listing in-stock parts no longer available from the original equipment manufacturer, call 800-253-0515. If a part isn't found in the brochure's listings, PartsPort may be able to provide a custom-made version.

Short of that, you have the option of buying a whole machine. Along with providing components of large systems (as large as mainframes), Computer Merchants Sales, in Hawthorne, N.Y., buys and sells new and used equipment from a wide assortment of microcomputer vendors, including big names such as Apple, Compaq, and IBM. The savings can be considerable. (For example, on an IBM 33-megahertz 486 PS/2 with 8 megabytes of memory and a 160-megabyte hard drive, the current list price, if new, is $5,475; if used, $2,250. An IBM Quietwriter III printer, if new, is $1,815; if used, $225.)

To subscribe to the quarterly Computer Price Guide ($70), a 48-page catalog of such systems and components, call Computer Merchants Sales at 914-592-1060.