Huaqiangbei is the most well-known market in Shenzhen, dealing with various kinds of electrical and digital products. No need to have tour but go there by yourself. No trader would like to give as reasonable as possible price to his guests. So, you'd better have an idea about the good's price you wanted in advance. And, most businessmen ask for high price from foreigners, so don't be shy to cut down the given price~