The basic need of adventure is supply, and perhaps one could say the basic goal of adventure is treasure. In Seed, characters share an inventory among them, consisting of everything at their disposal - healing potions, dangerous explosives, weapons both mundane and magical, and the armor that stands between hero and monster.
There are two broad types of items that characters make use of - "Equipment", dependable tools of war, weapon, armor and occasionally shield. Characters equip armor and receive defensive properties, and they wield weapons to determine their physical attack mode.
There are also "Usable Items" - Items are disposable and usable. Potions, antidotes, and that sort of thing, and also rare reagents that cast magical spells. Some items exist to work with action abilities - Samurai require ornate ceremonial katana to command sword spirits, Ninja hurl shuriken, Chemists possess an arsenal of mixtures only they understand.
As characters adventure, they will amass wealth, typically in the traditional Final Fantasy currency of "Gil". They can then purchase items from the item list and their equipment list, according to the outfitting system in use in the game, whenever they visit a store.
Just as characters have a "Tier", so do stores - throughout any item list, every article has a tier listed next to it, which is how advanced the shop must be before it will have that item in stock. Seed generally expects that shops will advance at the exact same rate as the characters do, so a character will always be able to purchase gear and supplies of their tier and no higher.
In a traditional Final Fantasy story, the advancement of shops is modeled by characters moving from town to town - so that every tier, in a way, would come with a travel to new horizons. Shops could also advance by receiving new shipments from abroad, by the craft that puts them together advancing, or by the characters receiving authorization to receive more strictly regulated equipment.
Some items are "Relics" - these are rarities that can never be found in stores, only stolen or dropped by monsters, found as treasure, or occasionally given as a gift - in this manner, they only appear as the GM desires. They can be centerpieces of the story or rewards for the observant, and their characteristics range in power from the appreciable to the overwhelming.
Item List - List of usable items which may be purchased, found, and used by the party.
Using the Inventory
Characters can use items in combat using the "Item" action, drawing them from the party inventory in a flash and applying it to a target. They can also freely use items outside of combat as well.
The inventory is a mysterious thing. Traditionally, it's something that's never looked at too closely in the video games, which depend far less on suspension of disbelief. In Seed, you can have this unexplained mysterious shared pool of items which can be accessed from any distance, or you can say "In fact, we are actually carrying these items. We're just simply capable of handing them to each other without taking an action, so it works the same way".
Of course, it's also important to keep track of it. If you're playing around a tabletop, it's easy enough to keep a sheet of paper with your inventory on it, crossing out numbers as the stock depletes and writing in the new value. If you're playing over the internet, it might be a good idea to keep a publicly editable page with a running tally, a separate room with a summary of what's been used and gained, or to appoint one player as the "quartermaster" and watch over item usage like a hawk.
Outfitting Systems
Outfitting systems determine what sort of equipment the characters have access to, determining their mode and power of physical attack and granting them special properties.
Equipment System - Wield a variety of weapons and armor with unique effects.
The Equipment System is probably your baseline expectation for how equipment works in a console RPG. Its wide varieties of weapon types allow characters to fluidly change up their tactics as they learn new abilities and enter different combats, and the unique equipment available at each tier causes combat options to flux in and out of prowess. It also provides potent, game-changing accessories!
Remodeling System - Characters use signature armor and outfits which they upgrade using loot salvaged from enemies.
Based on later-series games like VIII and X, the Remodeling System allows characters to piece together potent combinations of special properties throughout the campaign. Though characters generally adhere to one fighting style, they have the satisfying ability to adapt to whatever their adventures bring them up against after a few fights. …Just be careful with the Deadly Loot, okay?
Bare System - Characters don't deal with gear as a separate mechanic - attack modes, defenses and special effects are based entirely on the character's Job or Class.
The Bare System is an outfitting system that gets out of the way and lets you worry about other things. Characters don't need to spend time keeping their gear up to date - they're always ready to kick tail, possessing a small list of special properties they can switch between freely.
Regardless of system, all pieces of equipment refer to a common list of Equipment Properties and may also use other Properties.