Nikkei rebounds as investors buy on dips; Greece, China in focus

* Retail investors find buying opportunities after market dipped - analysts * Prolonged weakness in China market could become real risk to Japan - traders By Ayai Tomisawa TOKYO, July 7 (Reuters) - Japan's Nikkei share average rebounded on Tuesday as investors bought stocks on the dips, but uncertainty over Greece and China's efforts to rescue its slumping stock market capped gains in Tokyo. The Nikkei share average gained 1.4 percent to 20,399.82 by mid-morning, after tumbling