Along the lines of communication, but on a different front:
In some leagues you will find that there is little communication during the season between managers, as far as trade offers, notes about unusual occurrences in games, etc; in others, you will find a relatively chatty group. It just depends on who is in the league. Some managers will make a lot of transactions, some will make almost none.
Whatever type of manager you are, I would suggest that honesty is the best policy.
For example: sometimes during the course of a season, another manager may inquire about trading for one of your players. If you have been able to figure out which season that player is on, or been able to narrow it down, you can tell that manager which card you think they are on if they ask you. You are not obligated to reveal that info, but I have found that most managers will tell you if they know, as a courtesy and in the spirit of fair play (keep in mind: there is no reason why any of us can't research players on other teams and figure that information out for ourselves... and ALL of us have made mistakes and misread an injury reveal or a player's tendency... it happens).
Just be fair about how you communicate. Every one of us approaches the game differently, so what I think is a perfectly fair trade offer may seem like a totally lopsided and unfair one to the person I offer it to. All of us are trying to improve our teams, but none of us should be looking to deceive or rip off another manager, and I have rarely seen that happen. Just be consistent about it: although some follow the policy of not saying as much to teams in their own division as to the other 8 teams (why help a division rival?
), I would say that I don't recall the last time I came across a manager who, for lack of a better word, lies to other competitors. We are competing... but in a fun and friendly way, hopefully.
One side note: If your team drops out of playoff contention, don't have a "fire sale": it is not in your best interest to do so in any sense whatsoever.