One of the best gaming weekends in Ohio since 2010. Search facebook for "BOGA Weekend Retreat" for more info!
You should notice that "build" and "buy" in Le Havre are two very different things. Build is an action that you do using resources, and by putting your worker in one of the buildings that allow you to build. Buy (and Sell) are never actions. In your turn, before or after your action, you can build and sell as much buildings you wish, using francs when buying and receiving half amount of francs when selling.
I'm curious what the OP said that made you think this needed to be explained. It looks to me like they clearly understand the difference between buying and building.