Why there are so many trades happen and so many items are sold everyday? The reason is simple and obvious. Items are easy to get and selling items is easier than selling a wow account. There are many ways to get items.
While you are playing, you can get items almost you killed each animal, at least one. The more you killed the more items you will get.
Other players may have not pick up these items after they killed animals. So you can easily pick them up and do not consume your value of life.
But the capacity of your warehouse is limited. Only 23 pieces of items can you put in it. So you have to sell them out if the warehouse has no more vacant place.
So items are sold frequently in wow account. You can sell them to a vendor immediately and also can put them to an auction house. If your items are rare and valuable, you also can sell them in a wow online shop, such as farmer100.com.