For much of the last nine years, Zillow and Trulia have competed in the online real estate listings market they helped create. But after a speedy six-week courtship, the two are set to combine forces. Zillow agreed on Monday to buy Trulia for about $3.5 billion in stock, creating a giant repository for online listings for real estate and home values. Under the terms of the deal, Zillow will pay 0.444 of one of its shares for each share in Trulia. Based on Friday’s closing prices, the takeover bid is worth $70.53 a Trulia share, a premium of roughly 25 percent.