Magic items are rare, unique, and "the" vs. "a". By that I mean that when you find "the" +1 longsword, you have found THE +1 longsword. There isn't such a thing as a "generic" +1 longsword. There may be a couple in the world but each has its own properties and history.
The core rules do include more information on magic items. Including this part... Ire of the Gods
Magic items tear the veil that separates man from god; they break the rules. It is generally unwise to draw the attention of higher-order beings, lest their scrutiny find you wanting. A character who wields a magic item may suffer a penalty to his Luck score. This is a temporary penalty that remains in place as long as he wields the item. Generally speaking, the following actions draw a Luck penalty:
• Wielding a magic item of potentially sufficient power to challenge a demi-god. This is considered to be any rod or wand, any weapon of +2 enchantment or better, and any extremely powerful staff. Certain unique items may also qualify.
• Wielding any sort of magical artifact.
• Wielding any intelligent magic item, or one that possesses (or traps) a soul.
• Wielding a lower-powered magic item that is not of the possessor’s own personal casting.
The Luck penalty ranges from -1 to -4, depending on the nature of the item and the degree to which greater powers feel threatened. The final penalty is determined at the judge’s discretion. Some magic items may have arcane powers that partially offset this Luck penalty, but that is rare.