Want to Buy a Stock Market?
Edward Wyatt (Market Watch) column on next month's decision by National Association of Securities Dealers to consider issuing Nasdaq Stock Market stock to some of 6,000 brokerage firms that make up the association and 5,000 companies whose stocks are traded there; says Nasdaq shares may later be traded to public; says stock could be one of most intriguing investment opportunities of next decade; graph (M)