Each piece of equipment should include a checkbox, instructing the game to maintain a specified amount of inventory as ground mission retrievals and available funds allow, automatically purchasing the checked equipment should current inventory drop below the specified amount, or alternatively, to refrain from auto-selling goods needed to maintain the specified amount.
For example, if I set fragmentation grenades to maintain inventory at 20, and used 12 of my current 24 grenades on one mission, the game would immediately purchase 8 grenades in order to bring the inventory back up to the specified 20 automatically (as funds will allow, if funds are insufficient, the game will inform me).
In the case of auto-selling, if I instructed the game to maintain an inventory of 40 plasma blaster rounds, set the blaster rounds to auto-sell, expended 7 of my 42 plasma blaster rounds in a mission, and retrieved 14 from that mission, it would only sell 9 of those rounds, as 5 are needed to achieve an inventory of 40.
This enables a player to avoid the hassle of manually replacing items, or having to disable auto-sell and manually sell off excess gear.