There are two schools of thought regarding rules transparency. One school holds that the players should know the rules of the game, at the outset, to ensure that they have all the information they need to make good, in-game choices. Another school argues that the rules should be discovered, in-game, or should be common-sense rules that the players should know from real-life anyway.
Let me give you an example of a non-transparent rule. One of the rules I have often applied, but have not revealed to the players, is that the cost of an item is often, though not always, a good indicator of quality. This is a common-sense rule, but one that I do not explicitly reveal to the players prior to the game.
Let's apply that rule to the purchase of weapons.
In my games, second-hand weapons that are sold cheaply are often poorly made or badly maintained.
For those weapons that are new, but sold cheaply, it may be because the Weaponsmith knows they are brittle or soft, and so discounts the price of those less sturdy weapons. Alternatively, the weapons may have been made by one of her apprentices, whose skills are not up to the same standards as the master Weaponsmith.
Players will often want to haggle with the NPC Weaponsmiths and shopkeepers, to pay the least they can for their weapons. It can be tempting for the Players to purchase weapons that are cheaper, thus saving their coins for other purchases and activities.
Most weapons are worth between 3 and 15 coins. Light weapons that do 1d4 damage cost 1d6 coins. Medium weapons that that do 1d6 damage cost 2d6 coins. Heavy Weapons that do 1d8 of damage cost 3d6 coins. Tremendous weapons inflicting 1d10 damage cost 4d6 coins. I often allow the players to purchase whatever weapon they desire, designating what level of damage the weapon does. The player can then dice for the cost of the weapon, advising me as to the type and weight (damage level) of the weapon, and final purchase price.
Once the purchase price and weight (damage level) of the weapon is known, I roll a number of addition d6 so that the total number of dice rolled is 6. So for a medium weapon that costs 2d6, I roll an additional 4d6. I add the cost of the weapon, and my roll, and consult the following chart.
Weapon Quality Table
Score : Condition : Notes
6 – 11 : Flawed Weapon : breaks on first combat use
12 - 17 : Poor Quality: breaks on roll of 1, 2 or 3
18 – 24 : Average Quality : breaks on a roll of 1
25 – 30 : Excellent Quality : weapon gets saving throw on roll of 1
31 – 36 : Masterwork : roll 2 dice, weapon does higher damage
The result of this table (particularly for those more expensive weapons) is that players who pay very little for their weapons are more likely to find out the hard way that they got what they paid for. I do not reveal the above game mechanics however. Those players who decide to purchase items on the cheap will have to find out, during the game, that weapon cost and quality often go hand-in-hand.