What do you plan to do with that dusty 486DX computer cluttering your closet? Head to OnLine Exchange (http://www.uce.com), run by United Computer Exchange (UCE), and put it up for sale in a global Internet marketplace. How will you collect your cash? UCE handles that. For a fee based on a percentage of the sale (commissions vary from 10 percent to 15 percent), UCE advertises your computer's availability, then if you like an offer, UCE clears the credit card or check. In the meantime, you send the computer to UCE. You then get your cash, and the buyer gets the computer. It's a deal that takes a lot of the fear and uncertainty out of Net buying and selling.