CoStar Group, a commercial real estate analytics and marketing firm, sold the building that houses its Washington, D.C., headquarters this week for $101 million, which is a 146% gain on the price the firm paid for it. GLL L-Street 1331, an affiliate of Munich-based GLL Real Estate Partners, bought the 2-year-old building with cash in a deal expected to close later this month. CoStar with more than 1,000 employees will still occupy the majority of 1331 L St. after the sale is complete. The firm will put $15 million in escrow to fund additions and improvements to its space and...