First, move all the cargo to the cargo tab. put a seporate window in the tab that houses the cargo you can buy.

Second: Subtabs for the buy menu

Weapons Engines Batteries Ships



It would be easier for navigation of the menu

I think that buy, sell, and ship configuration should be on a single page. With good design, this should be possible.

Drag and drop between areas would be ideal. If that's tricky, a small row of buttons customised to what's selected would do the trick. i.e. if a shop item is selected, show "Buy", and if an inventory item is selected, "Sell".

If a slot on the ship is selected, only show shop and inventory items that fit in that slot. If a shop item is also selected, show the additional buttons "Equip" (which would buy and automatically equip, swapping out any item on the ship) and "Trade" if there's an item in the slot (which would sell the item on the ship).

If an inventory item is selected at the same time as a ship slot show the additional button "Equip" (which works the same as the shop version, only it doesn't cost money).