It is funny because I never notice this concept until higher level. As you level the most guaranteed item you will get is cloth. Cloth is a huge seller needed in professions and recipes. You also may pick up green items, quest rewards you cannot use and vendor trash.
From the picture above you can see I circled the item I chose because it was worth 24 gold and it was not a good enough upgrade for me. So really I earned 29 gold from that quest when you include the quest reward of 6 gold.
If you pickup skinning, mining or herbalism you can make some good gold. Starting out you may not make a whole lot but later you will. In most situations the higher level items you farm always bring in more gold, unless it is an item of useless value.
There always is people to lazy to farm so they will buy these items. Just because you have crafting professions it doesn't mean you need to farm all the time. While doing your regular quest you will kill plenty of humans and animals. While questing it will be easy to run into mining nodes, plants to herb and animals to skin.
You can mouse over items in your bags and the price of each to sell will show up. On occasion you may even get items with values of 1 gold that are trash. So make sure to keep an eye on what you get during questing and sell to vendor later.
This is just a few tips outlined for people to remember when they quest. It will help with giving you some gold or silver.
