Do the supreme court justices serve for life?

Yes , They do.

Explanation:

There are only two events that can overturn this.

First is if the supreme court justice personally stepped down from their position.

Second is if the supreme court justice receive an accusation that prove they are not in a 'good behavior'. In that case, the senates will review the accusation and vote among themselves to whether strip the supreme court justice or not.

the government i believe.

b) to encourage free trade