Aside from rules such as "no advertising" the underlying principle of all of the rules is that: a character (note a "character" and not a "player") may only gain an unfair advantage over other characters by in character (IC) means.

I think your question revolves around what is "unfair" since there doesn't appear to be any question about the fact that some amount of OOC knowledge is shared from character to character of the same player

Of course "life isn't fair" and what is "fair" is very subjective, but as a practical matter, the Immortals have a duty to keep the playing field as level as possible. My interpretation of "fairness" in this context is based on what information can it reasonably be expected that a large majority of the players will possess.

For instance, I think it reasonable to assume that most players know where to get a jewelled broadsword and Cloak of Nightwing. Thus, while this OOC information is giving your character an advantage over a newbie who only knows of TV guards' eq, it's not "unfair" per se.

Intricate quests that were meant to challenge a player and the character, by their very nature, will not be known by many players. Thus, transferring such information from one character to another can easily be considered "unfair" to the vast majority of the other players who do not have access to such information.

Those are the extremes, the real questions come with the examples that fall in between. The Immstaff doesn't have a formula; we use our best judgment about what is beneficial to the game and fair to other players of the game. Chances are that the greater the reward one gets from the transfer of information, the less likely we'll be to condone it. If you're unsure whether you can use information gained with a previous character, I would suggest you play it safe and don't. If it's really agonizing you, email the Imms.