I guess it would work much like the current system. When you sell a module it is available to buy back until you leave the station or log out. I really wish the there was some module storage function, would make it easier to swap out modes for some of my ships, like my Python, it gets used as a battle ship with shields, a SCB, hull reinforcement etc, but I also use it for some trade missions as well. Would be nice if I could store my unused modules rather than having to sell them as my home port doesn't sell all of them or at a high enough class.