This is the first installment of a new weekly real estate column from Mark Wellborn of UrbanTurf, a local blog and newsletter that covers the residential real estate scene in D.C., MD, and VA. A few weeks before the Democratic National Convention came to Denver this past summer, an email popped up in my inbox from an industrious friend in the Colorado city. “I am going to get $3,000 for my place for the week,”…