Magical items fall into three categories of uniqueness:
Common items offer useful but limited abilities. The processes of their manufacture are well known, and anyone with the money can track one down for purchase. Large cities will almost always have shops where you can purchase common items.
Common items lack activated powers. They usually confer only a simple bonus or a static effect. For example, a pair of gloves that grants a +2 bonus to Thievery checks is a common item.
Common items sell for 20% of their listed gold piece value. About half the items you find on adventures will be common items.
Uncommon items have powers, but these powers are typically daily abilities. They have static effects, but they are rarely character-defining or critical to a hero’s identity.
Uncommon items must be found. They are seldomly available for sale and few people know how to craft them. Even those smiths who can make them require exotic, difficult-to-find materials to complete them. Uncommon items sell for 50% of their listed price. A little less than half of the items you find on adventures will be uncommon.
Rare items are the most powerful items. Rare items tend to be “character defining” and will often make all the difference in a tough fight. Basically you’ll know them when you see them.
Expect to receive roughly one rare item per character per tier. Rare items are meant to be character-defining, powerful objects that help forge the character’s identity in the world. If you find a flame-tongue weapon, you’ve uncovered an important, powerful blade.
In general, you won’t be able to create or buy rare items. They are simply too hard to find to show up in the hands of a merchant or trader.
Rare items sell for 100% of their listed gold piece value. If you find one and want to sell it for cash, you’ll have no trouble finding a buyer willing to pay an exorbitant price to take it off your hands.
(the info presented here has been distilled down from this article.)