"Michael Lewis article on Jonathan Lebed, first minor ever to face proceedings for stock-market fraud, and murky questions case raises about what exactly constitutes illegal market manipulation; in September 2000, Securities and Exchange Commission settled case against 15-year-old, who used Internet to promote stocks from his bedroom in New Jersey suburb of Cedar Grove; Lebed agreed to hand over $285,000, but SEC did not go after additional $500,000 in profits Lebed accumulated in six months of trading; Lebed was accused of 'artificially' influencing market by posting hundreds of messages on Web touting stocks he had purchased; however, 'artificial' appears to mean whatever SEC says it means, or what it can persuade courts it means, highlighting illusion that stock prices are ever 'real' and fact that they are continuously influenced by media, professional analysts and companies themselves; photos (L)"
