The couple sells the house and then wants to do something else with the money they get back. Maybe they buy another house or give money to their children or both. They still want to do those things, but they have $1m less from selling the property than they expected, so they liquidate $1m in savings which they wouldn't have otherwise needed to do. Alternatively they spend $1m less, or invest $1m less, which is $1m less revenue or capital for people and businesses in the rest of the economy. Either way it's not good.

It's a bit simplistic, but the point is there is a real loss of wealth in society. If the house is sold the new buyer got a bargain, but they probably just got more house for the same money spent on their side. That's good for them, but on that end of the transaction it's likely to be roughly economically neutral, in terms of the whole economy.