Sorry to break it to you guys, but after doing extensive research (ok, maybe not-so-extensive) I found that most trades are generally done, in fact, in the offseason period. Consider these numbers, from the 2010 season:
14 trades in April
2 trades in May
1 trade in June
1 in July
9 in August
9 in September
2 in October (The Marshawn Lynch trade, and the Mike Bell/Jerome Harrison trade)
The trade deadline generally is in the 6th week of the season, or Mid-October
What this shows is that most trades occur around the beginning and ending periods of the off-season. In the beginning, where teams are trying to make speculative adds on talent, and the end, where teams are trying to fill out rosters (though a lot of the trades, all in all, were for future draft picks - another good reason why having them added to Madden improves the franchise mode).
April was generally around draft time, as there were a couple draft-day deals listed, as well as trades for picks before the draft.
The heavy activity in August/Sept. was generally the end of August/beginning of September, or when a new year in football would begin in Madden. So basically, the CPU offering trades in the off-season in Madden would be enough, depending on the volume in which it happens.
All in all, there were only
TWO (2) trades in the regular season. Most roster additions were from preseason cuts and signings, which is ANOTHER reason why the expanded rosters were well needed. If you're on your job, you can expect to be scouring the waiver list for ways to upgrade you're team.
It would be nice if the CPU did trade during the season, but why add it if it doesn't work? I'd rather have zero CPU-CPU trades than 5-10 unrealistic ones, such as trading Asante Samuel for Vernon Davis or something. The only trade from the entire year that had a true impact was the Marshawn Lynch trade; however, if Lynch was on ANY other team, he would've been traded on draft day, as the Bills, for some reason, couldn't pull the trigger.