After some update, you do not need a ship at a haven in order to trade for resources. Coupled with the fact that you get more food than you pay mats, and more mats than you pay food you can buy food, then mats, then food, then mats until no haven has any offer left. Repeat daily for infinite resources.
One of those should be a bug. Either you need a ship at the haven or you cant flip for profit.
I never noticed that you can flip … I just financed a helios with this and now I feel dirty