Online real estate veterans Jamie Glenn, Sean Black and Karan Sakhuja have launched a new consumer real estate business dubbed Knock. It takes a page or two from Opendoor, which is innovating in the online real estate space with homeseller guarantees. Here is it how it works: Consumers come onto the site or app and request a “price guarantee” to sell their home. Knock then offers a price for the home. If it is accepted by the seller, Knock takes professional photos, aerial video of the house’s exterior and a full-immersion virtual reality video tour. Knock promises to pay to promote it to millions of buyers searching the local MLS, Zillow, Trulia, realtor.com and Yahoo. A screencap from the Knock website. Whether you accept an offer from a buyer or take Knock’s…