Opendoor, an online real estate start-up, unveiled a new service this week that lets sellers trade-in one home for another one instead of watching it sit on the market or being forced to juggle two mortgages. The San Francisco-based company launched the program in Phoenix and Dallas with a goal to make buying and selling a home “as simple as trading in a car.” Once sellers find a new home, they go to the Opendoor website, decide on their closing date, and the company will take it from there, including dealing with the new home’s real estate agent along with the title and escrow. To simplify the transaction, the company doesn’t...