Tesla Inc. shares fell 7.5 percent in premarket trading on Monday as investors prepared for its chief Elon Musk’s proposed sale of about a tenth of his holdings in the electric-car maker following his Twitter poll. Musk, Tesla’s chief executive officer and the world’s richest person, tweeted on Saturday that he would offload 10 percent of his stock if users of the social media network approved the proposal. The electric-vehicle maker’s shares were down 6.2 percent at $1,146.43. Shares listed in Frankfurt were down about 7 percent at 989.10 euros. The Twitter poll asking Musk’s followers if he should sell stock garnered more than 3.5 million votes, and 57.9 percent of the people voted “Yes.” Musk had previously said he would have to exercise a large number of stock options in the next three months, which would create a big tax bill. Selling some of his stock could free up …