Inventory is determined when you first arrive at the store node

The store inventory is generated when you jump to the store node on the map. Revisiting a store node that you’ve already visited will not change the inventory of that store.

I tested this by doing the following

• Start the game
• Proceed until you see a “store” node
• Save and exit
• Go to My Documents\My Games\fasterthanlight and make a copy of continue.sav
• Start the game
• Visit the store
• Exit the game
• Restore continue.sav
• Start the game
• Visit the store

On the first visit I got a store containing the following;

On the second visit I got a store containing the following;

Since these two inventories are different, and my save was from before visiting the node, the store must be generated as you visit the node.