SSBA

Matching Salary:
When trading players, unless you are under the salary cap, the total salary you receive (this years salary, the length of the contract has no relevance) must be within 15% of the salary you send.  If you are under the cap, you need to have enough salary to be able to "absord" the difference between the amount you receive and the amount you send.  Draft picks have no value.

This can get complicated.  The easiest way is to use this spreadsheet I made:
http://ssba.phillyarena.net/files/trade_checker.xls

Simply fill in your caproom and player salaries on the left, then the person you're trading with's caproom and salaries on the right, and it will tell you if the trade works.

When Can't Trades Occur:
- Trades cannot be made after the trading deadline (Day 100).  Once the playoffs have ended, trades can resume.  Trades can be made at any other point during the simulation.

Who Can't Be Traded:
- Players signed for the MLE or LLE can't be traded.
- Players signed or previously traded can't be traded until 60 days have passed since their signing/trading, except when they are the only person being sent out by that team in the trade.

Draft Picks:
You can never fail to have a 1st round draft pick 2 years in a row.  So, for example, if you trade your 2006 first round draft pick, you cannot trade your 2007 1st round draft pick unless you acquire somebody elses 2006 first round pick, as you then wouldn't have a 1st round draft pick in 2006 and 2007.