The pilot reported that he performed a "runup" and switched the fuel tank selector to the fullest tank prior to takeoff on a cross country flight. During takeoff the pilot retracted the landing gear, climbed to about 300 feet above ground level, and then encountered a loss of engine power. He stated that he banked left to avoid housing and landed the airplane at an intersection with stopped traffic. An airframe and powerplant mechanic that assisted the first responders stated that he found the fuel selector switch in the four o'clock position between the three o'clock right hand tank position and the six o'clock off position. An on-scene examination of the airplane revealed no other pre-impact anomalies.