The "Quality" percentage that each weapon displays is essentially a combination of the weapon's Damage and Accuracy, with a score that reflects the range of it's potential in each stat. 50% of the weight is assigned to the Damage stat and 50% to Accuracy.
Take the following example: A weapon has Damage that can range between 30.00 and 31.50, and Accuracy that can range between 50.00 and 55.00. The weapon has 30.75 Damage, 52.50 Accuracy and no special bonuses. This weapon would have a Quality rating of 50%.
According to Weapon Changes Number 2, Accuracy is still represented by a 0 - 100 score. Values below 50 represent a negative adjustment to hit-rate, whereas above 50 represents a positive adjustment. These values continue to represent the exact hit-rate you'd see when your speed matches your opponent's dexterity.
Accuracy has less of an effect when the initial hit-rate (created by your Speed vs opponent's Dexterity) gets closer to 0 or 100. I've added some examples below to show you how it works.
If the initial hit-rate is 10%, 60 Accuracy will increase it to 12%
If the initial hit-rate is 25%, 60 Accuracy will increase it to 30%
If the initial hit-rate is 50%, 60 Accuracy will increase it to 60%
If the initial hit-rate is 75%, 60 Accuracy will increase it to 80%
If the initial hit-rate is 90%, 60 Accuracy will increase it to 92%
You will notice when purchasing a weapon or piece of armor that it will give you a rough of idea of how much weapon damage and accuracy it will offer. The higher the better, but like mentioned in Kobie’s post, it all depends on what the person are interested in paying. Also, referring to other know weapon and armor salesmen would give you a great idea.
If you have anymore questions please feel free to message me.
Reference 1
Reference 2