There’s no gold in PoE. Items are sold to and bought from players and NPCs by trading for a variety of currency items.
Any loot you find but have no use for can be sold to a vendor. Vendors will trade you Scroll Fragments, Transmutation Shards, Alteration Shards or Alchemy Shards for it, depending on the rarity of the sold loot. These shards and fragments are of low value, but can be combined into orbs and scrolls that have a use in crafting items.
All currency items can also just drop randomly as world drops. These are the most important currency items for beginners:
- Portal Scroll: Opens a portal to town
- Scroll of Wisdom: Identifies an unidentified item
- Orb of Transmutation: Upgrades a normal item to a magic item
- Orb of Alteration: Changes all the modifiers of a magic item
- Orb of Augmentation: Adds another affix to magic items with only one prefix/suffix
- Orb of Alchemy: Upgrades a normal item to a rare item
- Orb of Chance: Upgrades a normal item to a random rarity (magic, rare or unique)
- Chromatic Orb: Changes the color of sockets randomly
- Jeweller’s Orb: Changes the number of sockets randomly
- Orb of Fusing: Changes the number of links between the current sockets randomly
There are many more Path of Exile currency items. See the currency page on official website for a full list. It is advised not use any currency until you know its value, since many rarer orbs are quite valuable in the late game to trade for items with other players.
The best weapons are often crafted with the use of currency. Example: find a high base-quality type white weapon, increase the quality to the max of +20% with Blacksmith’s Whetstones, then apply an Orb of Alchemy which will upgrade it to a rare weapon, adding a random number (between 4 and 6) of different mods to it, which can increase its power significantly.
The most valuable/rare piece of currency in the game is a Mirror of Kalandra. Their drop rate is approximately 1/5,000th of the drop rate for Exalted Orbs, which are already rare drops and are used as high tier currency for trading with other players.
The least valuable/most common piece of currency in the game is a Scroll of Wisdom. Expect to obtain several hundred of these eventually. And all of the PoE Orbs can be bought on GM2V official website.