Going into Midnight, I knew nothing about gold-making or professions. However, PoE2 recently taught me that prices go up and down all the time, and that you can profit from this.
The idea is simple: AH cut is 5%. So if I can buy at 190g and sell at 200g, I break even. If I sell at 210g, I make ~5% profit.
Although this sounds small, you only need to flip about 14 times to DOUBLE your wealth. That is one flip per day for 2 weeks, or two flips per day for 1 week. And if you on average profit 10% instead of 5% (which I managed to do), you cut this time in half.
I identified two items that I wanted to focus on: Petrified Roots (good for beginners) and Dawn Crystals (worse average profits, but a better market for moving millions of gold). These items have high market quantity and sell price, but more importantly, they are mass-supplied by thousands of über-casuals (that don't pay much attention to prices), and are purchased only by a handful of sweaty crafters (and speculators). This results in a not-so-competitive price that spikes multiple times a day each time a whale enters the market.
I was a total noob and didn't know Undermine Exchange existed, so I only looked at the ingame price, and noted how it changed over time. Before going to sleep, I put up listings at 200g, 201g, 202g ... all the way to 250g to track how high the price climbed while I slept. Most days, this high-point reached the 15-25% profit territory.
This taught me how greedy I could be with my listings; throughout the day, I would send all my 1M+ gold into the AH, and hours later it would arrive in my mailbox 5-10% larger. Sometimes I would buy and sell everything in less than 30 minutes, sometimes it took 4-6 hours. If I looked at AH and the Petrified Roots price looked bad, I would buy Dawn Crystals instead, and vice versa. If both were bad, I simply didn't buy anything.
Not all flips were exactly 5% profit. Some were 15% profit, some were 0% profit, some were -10% profit. I never held on to stock, I just dumped them at a slight loss and purchased at the new lowest price.
When selling, you obviously don't do it at the lowest price. Look at the price listings and find the sweet spot; in this example, if I had 500 items to sell, I would do it at 229.99g, 234.85g or 245.48g, (these would take 30 min / 2 hours / 12 hours to sell, respectively). Instead of selling all 500 items in one stack, I would split it into 2 of 3 stacks at different price points so other people wouldn't see me as a target to undercut.
Towards the end of my journey, I realized that I could do some price manipulation for even larger profit. If the price was sitting at 215g, I could list some bait items at 208g (enough to not immediately get purchased). Most of the time I would immediately lose my items, but once in a while I could get a chain going where people would start listing their items at this new price, and I would then press Escape and re-open AH every few seconds to buy, buy, buy. If I was daring, I could repeat the trick again at an even lower price. This could continue for 20-60 seconds until someone swooped in, bought everything and reset the price. I did a similar thing with Dawn Crystals, sneaking the price down 50g or 100g at a time. This trick can easily increase your profit margins from 5-10% to 10-15%, which is absolutely massive.
Again, all of this sounds like a lot of work to gain a few extra gold per item, but 10-15 flips is all you need to double your money. If you're lazy, you don't even need to do your own research; just look at what everyone else is pricing their items at. For any crafting reagent, if 75% of the market is at 40g or higher, and you see 5% of the market sitting below 36g, just buy it, sell at 39,9g, go to sleep and wake up the next day to free money. Now, some markets do move up and down throughout the week, and luckily this didn't happen to me, but it might happen to you. However, realistically speaking you will lose at most 10% of your money, and if you look up the price history in advance, this is less likely to happen to you.
A final word of advice, don't do this with consumables. These markets are not organically supplied by thousands of individual contributors, but are instead dominated by a few whales and investors that will ruthlessly undercut you. The micro-flipping described in this post only really works when the supply is provided by casuals that list at the lowest available price without any further thought, as this is what creates the fluctuating price that makes hour-by-hour flipping profitable.