I think the Mercantile skill is a skill worth talking about. It's the skill of bartering with merchants to get better prices for the stuff you want to sell. How is this done? Well, you first have to get the merchant to like you better, which means you'll have to train up your speechcraft on them. How is this done?
Well, when you enter dialogue with a merchant, you have several choices to click at the bottom of the subject lines. The first choice looks like a face and that's what you want to click. You'll then get a circle divided into four wedges. Beginning from the top and going clockwise you have Admire, Joke, Coerce and Boast. When you place the arrow over one of those you'll see the person's face change in reaction to it. They will like and love two of them and dislike and hate the other two. Once you have that set, and with a little practice, you'll figure out how to raise their disposition to the maximum. This is done by selecting fuller wedges for liked/loved subjects and less full wedges for disliked/hated subjects. Believe me, with a little practice you will be able to train up your speechcraft pretty well. Oh, and the wheel always spins clockwise one wedge after you select one, so take that into consideration so that you don't inadvertently stick yourself with a full wedge on a hated subject. And once you get your speechcraft to Apprentice, you'll have a chance to spin the wheel once during your chat with them, just in case you get a wedge you don't like.
So, once you get the merchant to like you by maxing their disposition (I like to get it as near to 70 as I can, even if I have to bribe them a little), then you can haggle with them. Click the next icon over to bring up the trading menu and then click 'Haggle'. You'll get a slider showing you what the merchant is currently buying/selling at, along with their disposition. Most shopkeepers will start with a buying ratio of around 34, which is deplorable. If you're haggling with an expert haggler, try starting them at about 45. If they accept that, then sell off your stuff to them. If you have stuff stacked (like arrows), and you have three or more, you'll get a slider asking you how many you want to sell. Sell things one at a time, if you're patient. If you sell the whole bundle as one transaction, that is all your get credit for, one transaction, so sell things one at a time. Arrow bundles are fantastic for doing this. However, if you have hundreds of arrows in a stack, you may want to sell them in bundles of five or ten until you lower the number to where you're comfortable selling them one at a time. Believe me, if you've been through a goblin dungeon, then you're going to have lots of iron arrows.
The next time you visit that same merchant, you may try haggling again, if you sold them a lot of stuff the previous trip. When you click 'Haggle' you'll see that their disposition may have raised since your last visit, and if they're an expert, try going up no more than two more points. Eventually, if you keep selling them a lot of stuff (one at a time), you can get that slider around 70.
If you're haggling with a Novice, Apprentice or Journeyman, you can probably start out at around 50 with your first haggle and bump it 2-4 notches each subsequent haggle. Eventually you'll get them to around 70 or more.
Most general merchandise dealers will be Apprentices or Journeyman hagglers, and they'll have between 600 and 800 gold to barter with. Most Expert hagglers are the Smiths who deal in weapons and armor, and they have around 1000-1200 gold to barter with. There are a very few general merchants who have 1000 gold to barter with, which means that they're a better haggler than the ones with less gold to trade with.
Finally, most mages who sell spells are very poor hagglers, so you can raise their dispositions if you aren't a member of the Guild and then haggle them down to 60-70 on the first try.
Believe me, if you want to raise gold quickly, without using the console to give it to yourself, become a haggler.