Starting trade period after the first preseason game allows plenty of time to get trades done No Fa transactions should occur after season ends. Yahoo is up and running by this time
Not sure what Yahoo allows in the offseason but I would suggest not allowing transactions until draft slots and keepers are declared to avoid problems in particular with trades. In leagues with a limitted number of keepers Allowing teams to trade players b4 keepers are declared will essentially give a team the ability to trade players they would no longer have reason to expect value from.
Example A 2 keeper league a Team has RG3 Foster and D Martin maybe an A Morris would allow them to essentially get an extra couple of #1 picks for Martin or RG. Everyone else is playing for 2nd place from the 1st week of the season This makes it difficult to keep people interested if half the league has "no chance" to win and then the trades get progressively worse when teams give up on this season and start trading their only good player for next years draft pick
The trade deadline holds until keepers are declared no FA acquisitions until after the draft