If he wanted to manipulate market then just the rumor about the purchase would send stocks higher. In fact, after his announcement on April 14th, Twitter rose 7

If he wanted to manipulate market then just the rumor about the purchase would send stocks higher. In fact, after his announcement on April 14th, Twitter rose 7.5% on the next session (18th). No need to actually sign the deal to make few hundred millions of dollars. On the other hand, how would he cash out his gamble? You cannot announce that you want to buy 100% of the company stocks, and then start selling off the 9% that you already have, that would be a dead giveaway that the offer is not serious.