There are plenty of startups that say they’re making it easier to buy a home. There are fewer startups that are promising to make it easier to buy a home as an income-producing property. Among these is Roofstock, a four-year-old, Oakland, Ca.-based online marketplace where buyers and sellers buy and sell rental homes in more than 70 U.S. markets — homes with tenants residing in them oftentimes. The idea: both institutional and retail investors can buy and sell homes without f