Ameritrade's Datek Purchase at Risk From NASD Order Washington: The National Association of Securities Dealers ordered a unit of Datek Online Holdings Corp. to halt day trading by some customers, raising a potential obstacle to the company's acquisition by Ameritrade Holding Corp. Ameritrade and Datek must obtain NASD approval for a change in control at Datek. Ameritrade, the nation's fifth-largest Internet brokerage, plans to buy Datek, the seventh largest, for $1.3 billion in stock. The approval may be jeopardized because the association, which regulates brokers, has expressed concern that a Datek unit is permitting customers to engage in an illegal form of day trading known as ``free riding,'' the purchase of shares without the money to pay for them, according to a filing by Ameritrade with the Securities and Exchange Commission. Ameritrade and Datek say the unit, a business called IClearing that processes securities transactions, isn't doing anything wrong, and they will ``vigorously pursue'' efforts to prove their case. So what are datek doing? Are they not checking accounts before the orders are going in?