Gear and items are bought to be used in Thief 3. Don't go to great lengths to save items if you think they can help you stay alive and undiscovered, but don't waste them on lone guards or civilians who cannot fight back.

On normal, the maximum gear Garrett may carry is:

Health Potion = 10
Broadhead arrows = 30
Noisemakers = 5
Water Arrows = 25
Moss Arrows = 20
Gas Arrows = 5
Fire Arrows = 15
Flash Bombs = 20
Explosive Mine = 5
Gas Bomb = 5
Oil Flask = 5

If Garrett proceeds to pick up another item when he is at maximum capacity, the item is "picked up" but is not added to the inventory. Be sure you have room before you start grabbing items or you're just wasting hard earned loot!