There's three main forms of currency flipping.
Trade Sniping, Arbitrage and Swing Trading.
Trade Sniping
Trade sniping will NOT be covered in depth in this guide, as many players consider it to be unethical or even unconscionable conduct.
Trade sniping involves profiting from mistakes other players make.
Someone lists a Divine Orb for 130 exalts when 275 is the fair ratio? You snap it up and resell it.
Someone lists an unidentified or premium roll
Morior Invictus or
Prism of Belief at the price of a poor roll? You trade snipe it and relist it.
Someone posts a truly phenomenal rare item on Reddit that you know you could sell for 120 divines? You have your three Reddit accounts say "yeah it's not as good as you think but you might get 12-15 divines" then on your real account offer 20. (This is the nastiest example here; trade sniping can get worse and I won't disclose the nastier strategies)
Sniping will make you simultaneously wealthy and hated.
Doing so via flagrantly dishonest means may close some doors within the POE2 community, for example, potentially getting you added to scammer blacklists that are circulated by various guilds and discords. You may not even be aware that those doors are closed to you or why - negative consequences can be silent.
What defines trade sniping - rather that arbitrage or swing trading - is that the other party to the trade would regret trading with you immediately if they were knowledgeable about the game's economy.
I do not recommend engaging in this.
Fundamentally, this is not a PVP game. Loot monsters rather than other players, you'll have more fun that way.
Arbitrage
This is the main form of 'flipping' in POE2.
Imagine this scenario.
You are just getting started in maps, and you had a really good drop in your sixth map, a Greater Essence of Torment.
You have three choices: sell it instantly for 24 Exalted Orbs, list it for sale at 30 Exalted Orbs and maybe get a sale in the next 24 hours, or plan upon selling it when you have enough to sell a divine's worth at once, and forget about it.
It's not irrational to take the instant 24 Exalted Orbs here. Plenty of people do so. This allows them to make immediate improvements to their characters, helping find the next good drop sooner.
In the real world, arbitrage means buying in one market to resell at a higher price in another. For example, purchasing collectibles in Japan then reselling them in Australia where they are scarce.
In POE, there's only one market - but you can still be an intermediary between sellers with one of an item and buyers who want twenty of that item. Or between sellers who want currency right now, and a buyer who desires the item tomorrow.
Arbitrage trading is all about buying low and selling high on items that you do not expect to undergo wild price changes.
In the earlier example, arbitrage trading is being the person buying the Greater Essence of Torment for 24ex. You provide the seller instant liquidity, and then sit on it until you can resell it for 30ex.
Or maybe you elect to resell for chaos orbs or divine orbs instead, because you can get a better price there.
More on Arbitrage strategies later.
Swing Trading
This is much less of a thing in Path of Exile 2 than in real life markets.
The basic idea is that you sincerely believe an item will increase in price due to either an increase in demand, or a decrease in supply.
This can be done with currency items OR with items that can be equipped, although the latter cannot be done via the Currency Exchange.
For example - imagine you are watching Jungroan stream and he mentions a desire to build around a previously unpopular spell such as
Firestorm. You might conclude "Any Firestorm build probably desires a weapon crafted with Greater Essence of Fire and a well rolled or well corrupted
Fireflower amulet"
At this point, it's not unreasonable to start purchasing Greater Essence of Fire or
Fireflower with a view to reselling them later once hype is higher, after the demand spike. If you understand all the intricacies of how Vaal Orbs work, you might also pick up Distilled Emotions and Xoph's Catalysts.
Alternately, Ben_ might stream a strategy involving the Ritual mechanic and farming the access key for the King in the Mists encounter. You might watch this and be one of the first to conclude "if more people do this, that means more people using Ritual tablets and thus less people using Breach tablets - maybe this will make Breachstones more expensive".
In this case, you might conclude that trading for Breachstones is a good idea due to a potential future supply drop.
Or you might discover a completely overpowered build utilizing a little known unique and buy 72 of that item before going public with your findings.
The key difference between swing trading and arbitrage is that with swing trading, you have no expectation of reselling quickly.
And if you make a mistake in swing trading, you will lose more.