Groupon Inc forecast a surprise quarterly loss as it spends more to advertise its online marketplace, which is expected to make up for its lagging daily deals business, and its shares fell more than 13 percent in extended trading. Groupon, which competes with Amazon.com, eBay and Google, has been building an online marketplace, called Pull, that lets people search for and buy deals in their area. The majority of our customers in North America still have no idea they can come to Groupon and search among our 80,000 deals in real time, Chief Executive Eric Lefkofsky said on a conference call with analysts. However the shift to the Pull marketplace means that customers can delay their purchases rather than make them on the same day as was the norm in its daily deals business.