This is just over 20 silver in wool scrap profit looks like (after they sold in 10 minutes). It took around 2 minutes for the buy orders to get filled. Why do these items sell so quickly? Because they are only worth a copper to the seller! Super low profit margins like these work great despite the fact that they seem to be an insignificant amount of gold for the time invested.
Let's see, if I were to focus my trading post efforts on three markets like wool scraps (1 copper profit per flip), then I would earn 3 gold 60 silver profit per hour. You can easily play a bunch of markets in between checking every 10 minutes to repost buy orders and collect your gold, so this is by no means a cap on your gold per hour. Instead, it should be supplemental income.
Let's say that you play these 3 markets, 30 minutes a day, for a month. How much gold would you make? Over a hundred gold easy. But wait a second, you're going to take less than 2 minutes to create all your buy orders and list your auctions, so what should you do in the mean time? How about add another 7 markets? That way you're playing 10 markets and making 180 gold per month. That's just 30 minutes of your time! Imagine if you mixed in some speculation, mystic forge, armor flips, exotics, etc? Now perhaps you're starting to see how earning your legendary isn't so difficult if you diversify your markets between super fast 1 copper sellers, one day flips at 5-25% profit, and multiple day 20-100% ones.
The higher your profit range, the longer it will take to buy and flip your items. That's not to say you should avoid all high profit markets, but that you shouldn't tunnel vision all of your attention on them.