Great immersion idea, but before they add this, they need to fix so that we use ingame mechanics to find out where you can buy them...
I would also love it so that a busy station could get more ships and not so busy station would et fewer ships. and same for modules. and it would also be nice if ships/modules that usually are available but sold out to be shown as sold out. And ofcouse the ingame search tool to look for where to buy it, if you are at station that say they are sold out, they should be able to point to where they import them from or similiar. (this should then point to a nearby system that has that ship/module in stock)
On a side note, they already do this to modules, as they can be sold out a station, so you have to go somewhere else for it.