Bag in accounts wow is a container that your role uses to hold items. More and more wow players start to use tailoring skill to get a big bag, as larger bag can add available space to contain all of their gold and precious items.
Your backpack is a special bag that can not be replaced. And bags can be stored as a item into your inventory, which has nothing inside and not equipped (Bags in your bank slots are considered to be equipped) by the character. If you want to equip a lager bag, you can swap out a higher-capacity nested bag for the smaller bag. Don't worry, all of your items will be transferred to the larger bag.
Normal bags can carry any kinds of items, while those special bags can only carry specific kinds of items. Despite this, special bags can give the player more slots to store items than regular bags, and it is cheaper than normal bags of the same size. Each has their own benefits.
Before you going to the auction for buying a larger bag or begin to spam trade chat with buying a bag, you need check out what other wow players have. Even you can consider what bags the wow account has, which was bought from a website.
If you don't have tailoring skill, there is an auction for getting larger bags. Prices at auction go up and down everyday, maybe you can buy the cheapest bag which can go up to a very high price on the next day!