The U.S. Senate has voted to allow states to collect sales tax from online retailers, making it more difficult to buy tax-free products online.The Senate's vote of 69-27 for the Marketplace Fairness Act late Monday sends the bill to the House of Representatives, where it may face opposition from many antitax Republicans. U.S. President Barack Obama has voiced support for the bill, meaning he would sign it into law if the House passed it.