Some 560 million shares of Twitter will be available to be sold in the market today as the post-IPO lockup expires. Large insiders have stated they won't sell and Twitter CEO Dick Costello has said the company doesn't plan on a secondary offer. Both are attempts to fend off big selling. However, short traders have already been placing bets that many shareholders will sell today. The stock has dropped sharply over the past three months and many feel it is still overvalued.