@Mujo that's my take i have had a quick look at Costar from an investment point of view, and came to the conclusion REA is better placed. the US market is difficult and fragmented. Interestingly, Costar entered the Uk which put the skids under the leader there Rightmove which allowed the PE arbitrage for REA to make its bid. I'm doubly happy REA didnt take on Rightmove because now they would be fighting on two fronts. from what i can see Costar have not really made any headway in the Uk despite Rightmove being not as good an operator as REA, imo. REA is much more entrenched than Rightmove and Costar SP flat for 5 years tells a story of their success despite being reasonable operators. you need to be the leader.

Interesting that REA sought to takeover the dominant platform in the UK while CoStar acquired OnTheMarket which is not even the number 2 player in the UK (behind Rightmove and Zoopla). @Mujo You're right about CoStar going after the second best (or even third best).