Shares of Overstock.com Inc. were off more than 15% in afternoon trading Wednesday after Chief Executive Patrick Byrne disclosed in a filing that he sold 500,000 shares earlier this week. Byrne said in the filing that he sold 250,000 shares on May 13 at a price of $13.33 and sold another 250,000 shares a day later for an average price of $12.84. The shares are listed as being indirectly owned through High Plains Investments LLC. Overstock didn't immediately reply to a MarketWatch request for comment about the stock sale. Shares have fallen 20% so far this year, as the S&P 500 has risen 14%.