One of the biggest mortgage lenders in the U.S. said it would pay $1.75 billion to buy a well-known real-estate brokerage. The deal, once it goes through, could have major consequences for millions of home buyers.

The mortgage giant Rocket Cos. has agreed to buy Redfin in an all-stock deal valuing the online real-estate brokerage at $1.75 billion.

Rocket Companies agreed to acquire Seattle-based Redfin in a $1.75 billion deal that will bring together the nation's largest mortgage