Real estate portal Trulia will get all of its multifamily apartment listings from a single source -- Primedia Inc., the operator of a stable of rental sites including Rent.com and ApartmentGuide.com -- the companies said today. Trulia will also sell display advertising and other media products on behalf of Primedia, the companies said.Trulia CEO Pete Flint called the partnership with Primedia "a major step in solving the fragmentation of the rental market by linking Trulia's quickly growing, transaction-ready audience with premiere rental inventory."The new Trulia-Primedia ad network "enables us to offer our advertising partners access to one of the largest housing specific audiences online," Flint said in a statement.Trulia launched rental property search in 2010, obta...