Retail investors testified at the securities fraud trial of Andrew Left, pictured right, in Los Angeles. Bloomberg/Getty Images Andrew Left, a prominent short-seller, is on trial in Los Angeles for alleged securities fraud. A retired firefighter testified he lost most of his 401(k) after investing in stocks Left targeted. Left is accused of manipulating the market, but his defense says he shared his honest opinions. Federal prosecutors called a witness in the Andrew Left securities fraud trial who might resonate with the jury — a retail investor who said he lost his retirement nest egg by investing in stocks that tanked after being criticized by the famous short-seller. Billy Banks, a retired firefighter from Texas, testified at Left's trial in a Los Angeles federal court on Thursday.…