Quality weapons are those of exceptionally fine craftsmanship. The blade may be forged from the finest steel for flexibility and sharpness. The swordsmith may have carefully folded, hammered, and tempered the steel to a superb edge. The whole sword may be perfectly balanced, light in the hand, but heavy in the blow. There are many reasons why a sword or other weapon could be above average.
Careful craftsmanship and high quality give a weapon a bonus on the chance to hit or a bonus to damage. The bonus should never be more than +1. The bonus on the chance to hit is for those weapons that are exceptionally well-balanced, light, or quick. Weapons of perfectly tempered steel or carefully hammered blades gain the bonus to damage. The metal retains its razor sharpness, cleaving through armor like a hot needle through wax. Because they rely on mass and impact, bludgeoning weapons rarely gain a bonus to damage. Those that do get a bonus are because they have carefully shaped and balanced heads.
The quality of a weapon is not immediately apparent to the average person. While anyone using the weapon gets the quality bonus (even if they don't realize it), only those proficient in that weapon-type or proficient in weaponsmithing can immediately recognize the true craftsmanship that went into the making of the weapon.
Even then, the character must handle the weapon to appreciate its true value. For some reason, however, merchants almost always seem to know the value of their goods (at least the successful merchants do). Thus, weapons of quality cost from 5 to 20 times more than normal.
In your campaign, you might want to create NPCs or regions known for their fine quality weapons. Just as Damascus steel was valued in the real world for its fine strength and flexibility, a given kingdom, city, or village may be noted for the production of swords or other weapons. The mark of a specific swordsmith and his apprentices can be a sure sign of quality. Again, by introducing one or two of these (remote and difficult to reach) areas into your campaign, you increase the depth and detail of your world.