Dona Z. Meilach, author of Before You Buy a Used Computer, offers these suggestions to secondhand buyers: Join a computer users group for technical advice. Take along an expert to inspect the equipment. Make sure manuals are included and parts available. Find out whether the computer is under a service contract. If you buy long-distance, get a guarantee from the seller. Check prices of comparable new equipment. Make a list of what you need and stick to it.RESOURCES For leads: newspaper classified sections or national computer-buying newspapers, such as Computer Shopper, available at area stores or from P.O. Box F, 407 S. Washington Ave., Titusville, Fla. 32796. $1.95 copy. For recent prices of used computers and equipment: Computer Classified Bluebook, P.O. Box 3395, Reno, Nev. 89505. $30 a copy. For area dealers that buy and sell used equipment and names of local users groups: Washington-Area Microcomputer Directory, available at bookstores or New Local Resources, Inc., P.O. Box 2133, Silver Spring, Md. 20902. (301) 946-7712. $6.95 a copy. For demonstrations of new equipment and software: The Write Stuff, Computers and Word Processing for Writers, Nov. 3, 8 a.m. to 6 p.m., Shoreham Hotel. For information, Washington Independent Writers, 1341 G St., Suite 205, Washington, D.C. 20005. (202) 347-4973.