Hmm maybe I should make a merchant guide but heres some tiny info on blacksmithing since its the only main role atm for a merchant.
Basic Information:
Blacksmithing mainly makes weapons (not bows though) and heavy armor. As well as fortifying to increase maximum durability and sharpen to increase damage and lastly repairing as a blacksmith doesn't reduce the maximum durability as much as a merchant npc does. Later in there levels they can make platemail armor and near the end make the platinum/gemed version of the armor and weapons which is far far superior to the merchant bought ones (less speed penalties and more dmg or armor). They can also make tools to so you don't have to buy them off merchants and as well as some furniture but iirc only one or two items at max level.
Good information to know:
Blacksmithing has 3 main stats to it:
Intelligence: Gives you a higher chance of a craft to hit its maximum durability potential (Insanely helpful for doing work orders and apprenticeships).
Knowledge: Required for working with higher level material, makes it so you can get 2 ingots instead of 1 as well as lower chance of failing when smelting ore into ingots.
Faith: Increases your chance to successfully craft an item. I believe it's every 10 points is 1% higher success chance.
Blacksmithing and Mining go hand to hand so I will talk some what about it as well. Mining only has two attributes:
Strength: Inreases the chance of getting multiple ores when mining (does not increase the amount of gems one can get per mine)
Knowledge: I believe this one once you get it to a certain amount it increases the amount of ingots you get when you smelt ore. A different amount of knowledge is needed per type of ingot.
Some more information thats nice to know is work orders are randomly generated by what your blacksmithing level is and your carpentry. So if you have some carpentry skill, you will start seeing some work orders wanting tools or something that requires wood to make. Also never be afraid to throw work orders on the floor, most of them are never worth to make so keep getting more till you get some worth doing.
Leveling up and leveling your blacksmithing:
To level up first thing one would do is if you don't want to spend money on ingots is to find the ore. The best beginning spot would be to the north east part of the world map near ironforge is probably best. There you can find copper, iron, and coal. Collect about 100ish copper and other ores. Once you have a nice stock of ore lets go spend it. Find the closest smith or if your at ironforge the ore collector (if you want) and ask for a work order. Double click on the work order to see what it wants. If it wants anything higher than copper, red copper or iron throw it on the floor and get another. Once you have a good work order you like, go to the smelter (double left click it) and put in the ore you need. 1 copper ore = 1 copper ingot. 1 copper ore + 1 coal ore = 2 red copper. 1 iron ore = 1 iron ingot. Now make the ores you need and go to an anvil (double left click) and craft what the work orders want and drag and drop the item over the work order.
Some work orders and all apprenticeships requires the items you craft to be of a high durability (fortifying doesn't work). So basically you have to craft a bunch of the same item till you get a good roll to be able to turn the item in. The left overs is good to save if you get a work order that doesn't need a high durability or to sell to a merchant/player.