Billionaire Michael Krasny, the founder of the CDW computer-sales empire, on Friday placed his three-bedroom, 4,200-square-foot condominium unit on the 50th floor of the Park Tower on Michigan Avenue on the Gold Coast on the market for $3.4 million. In the early 1980s, Krasny, 71, formed what became CDW, and he took the company public about a decade later.