You can buy two packs from the quartermaster when you begin the Grey Warden Joining (once before you go in the wilds and once you come out). Once you have them, you should have 100 item spaces in your inventory. Collect everything during the main and side quests. When you find a merchant or someone possibly to trade, do it. Keep adding more to your inventory space. A pack is 10 spaces extra, but prices tend to vary from those you buy. The quartermaster in the beginning sells them at the same price. The dwarf merchant that follows your party to camp varies. The quartermaster at The Tower of Magi has one for a heavy price, but it is worth it because you can hold more items and sell more as you see fit. By doing this you can make a lot of coin by mid-game. What you buy and sell depends on the coin you get to carry through the game.