While the currency system in Path of Exile is quite brilliant and is one of the most innovative things that Path of Exile has offered to the online role-playing game world, there's no denying the fact that it can be quite confusing for beginners. Since there is no central currency and a barter trading system is utilized, the variety of currency items that are being traded back and forth for buying and selling Path of Exile items can get overwhelming for some. Therefore, a primer is necessary for new players to familiarize themselves with how economy works in this game.
Some currency items are more scarce than others, and their values are quite different from each other. Trading ratios are central to the understanding of Path of Exile's economy, so remembering the important ones is crucial to not getting ripped off when you're looking to buy stuff from other players and being able to make the most profit if you're selling something. As most MMO veterans would know, trading itself is kind of a mini-game in online role-playing games that follows a lot of real life rules and is an integral part of the gameplay experience.
There are 20 or so different currency items in Path of Exile, commonly known as Orbs. They are not just for trading, but also for crafting new items, so inflation is not that big of a problem. The trading ratios are dependent on the basic concept of supply and demand, so this game actually teaches quite a bit about real world economics and you may learn something if you do pay attention.
You can sell Orbs to vendors in exchange with other Orbs, and it's a good place to start if you're familiarizing yourself with the currency items in the game. You have to be careful though since some Orbs are actually worth more in player trades due to their usefulness, so you may want to not sell those to vendors unless you want to shoot yourself in the foot.
Three Jeweller's Orbs gets a Chromatic Orb, which is not worth the trouble unless you need Chromatics. Four Augmentation Orbs gets an Alteration Orb, which is useful for Augmentations. Two Alteration Orbs gets a Jeweller's Orb, and four Jeweller's Orb gets an Orb of Fusing, both of which are the same as the market value. An Orb of Fusing is at the same value as an Orb of Chance, but it's not worth selling to vendor since an Orb of Fusing is worth 4 Orbs of Chance in the market.
An Orb of Regret is equal to an Orb of Alchemy, which may not be worth it since Orbs of Regret are worth more than that in player trades since they're quite useful, going at around 2.5 Orbs of Alchemy for each one. Four Orbs of Chance for an Orb of Scouring and two Orbs of Scouring for an Orb of Regret are pretty much spot on in market value, so you can keep that in mind if you're looking to buy or sell Orbs of Regret.
As for trading ratios in player trades, they do jump around a bit depending on how much of a particular orb is in circulation and how much demand they're pulling in. It also has a lot to do with how many players are farming a particular orb, which can make it go up in supply, which can bring prices down.