Warren Buffett's Berkshire Hathaway is itching to do a massive acquisition, but is having a difficult time due to elevated valuations. The Oracle of Omaha explained his buying criteria for deals in his 2017 annual letter to shareholders released on Saturday. "In our search for new stand-alone businesses, the key qualities we seek are durable competitive strengths; able and high-grade management; good returns on the net tangible assets required to operate the business; opportunities for internal growth at attractive returns; and, finally, a sensible purchase price," he wrote. "That last requirement proved a barrier to virtually all deals we reviewed...