The Trade Broker and Inventory Selling are slow but steady ways to build up a fortune. Unfortunately, these aren't explained too well in the tutorial.

The Trade Broker

(click for larger image) When someone buys your item, you will be shown a screen like this with similar information, which I have numbered:
1. What item you sold. Mousing over the icon brings up information for the item. The rarity also appears here, as a color.
2. How much the buyer purchased it for. This is not your total profit made.
3. The fee. On every broker-sold item, 5% is deducted from your profit. In order to calculate how much you made, subtract the fee from the price.
4. The buyer. Red means offline, green means online, and they can be clicked to go to their profile.
5. The sell date. Less important, but hey, maybe you check the broker like you check stocks. If you check stocks. With event items, it's useful to be able to check sell prices per day.
I profited from the above exchange. How?

In the bottom right corner of the info-box for the relic, there is a number next to the gold symbol. 1856 is the base sell-from-inventory price. 2000 - 100 = 1900, meaning that I made 44 more gold selling this on the broker than from my inventory.
The Broker button in the inventory brings up this popup:

1. Lowest Price. This is what you want to focus on, as it's held as a standard when negotiating in Region Chat and if you want your item to be bought anytime from soon to ever.
2. Current Listings. How many of these are there? Demand (and, you know, a monopoly) might allow you to inflate the prices, except that other users can see
3. Lowest and Average prices for past sales, by clicking on the Item Info button under the green Purchase one. If there are no current listings, this is a good place to start considering pricing.
4. How many sales in the past 30 days. High numbers here means high demand, and therefore better odds of your item being bought (at a reasonable price).
5. How much you are selling it for. The one part of this information that you can affect!
That's most of what you need to know regarding the Broker. One last, sort of ettiquette note...

Nobody likes seeing pages of axes and ravens going for less than 200 gold, and it floods the broker while pushing back other items with value. While these are easily obtainable items with no sale price...I can't stop you from trying to get five more gold, but perhaps consider dumping it to help preserve the economy? One fairly common form of disposal is sending the valueless item to Krinadon. Recently, this was changed. Just sell from your inventory.
Next up, inventory-selling!

Selling From Inventory
The inventory-selling has no tax and gives the full value stated on the item, as well as being instant and not requiring you to wait for someone to purchase said item.

To sell, click on an item, as if to move it to a different slot, and then click an empty slot in the Sell area, highlighted in red.

Once the item is in the sell area, its image will appear in one of the sell slots, while its image in the ordinary inventory area will have a red background. On a computer, Ctrl+click also moves an item to the sell area. The gold amount at the bottom will increase as you add items, but in order to obtain that gold, you must click the [Sell Item(s)] button, which will become blue. The items will then be removed from your inventory, and the gold added to your balance.

And one more for anything I missed. You can post now.

Ah, thank you. Fixed.

Is it really a thing to send junk to Krinadon? And he doesn't retaliate by like ... flooding your inventory with ravens or whatever?

He's only ever done it the once, Kyber. :P But it's been a common thing for years.

It is a thing.
Er...
It was a thing.
You see, I kind of got a little...banned? (without getting blessed with a troll very sad)

After almost a week of sending him something cheap and random every hour I was online, but...just sell it, really.
(By the way, could you check if everyone's banned or just me, so I can edit the guide accordingly?)

Sure Ill try

Edit: I am unable to