A strong economy helped lift commercial real-estate sales to near-record levels last year, and many investors are counting on low interest rates to keep sales going in 2019. The value of commercial properties that sold for $2.5 million or more came to $562.1 billion in 2018, according to research firm Real Capital Analytics. That was up 15% from 2017 and only $8 billion shy of the $569.9 billion sales record in 2015.