I have to disagree, because the price free agents sell for are the real market value prices. Many teams with a lot of money would bid but they wouldn't overbid so those players most often were signed for the amount of money they are really worth...
I agree with other responses. The major problem with FA is that they are very often on the market when the transfer market is dead. They are overrepresented during a part of the season when few clubs are investing in expensive players, e.g. 3-4 weeks into the season, and during times of the day when very few are logged in, deadlines at 8 GMT is not too unlikely.
Managers with a player of similar strength as these FA usually consciously or unconsciously transfer list their players to not be sold when the market is low and for this reason it is mainly FA that daytraders can make bargains with. I would not sell a $25k salary guy 8 GMT as I know that I risk losing a lot of money due to the low activity in the transfer market.
And it is very difficult to buy players when you are not logged in. You do not really want to make a bid of $1.5 millions for a player that might only be worth $1M. If you are bidding at deadline you can much easier buy the player for something close to market value. Even if you consider market value to be $1M you do not want to make such bid on a player currently for $500000 so then you have problem with not logged in again. This means that managers able to log in at any time during the day can profit from this.
One way of solving this problem is to make sure that the FAs always are out on the market when many users are logged in. Of course it would punish people logged in when only few users online but they aer already punished today as most deadlines will always be at times with more users online.
The major problem with daytrading usually are that these players are bought and then transferlisted for a value even higher than their market value but if someone wants the player and has the money they might still buy the player. But it seems like the daytrading problem is not as big in BB as in e.g. hattrick, but at least I do believe that the limitation of FA is very good for BB. And then only to avoid giving too big benefits for people online 24 hours a day. The major successes should be from managing the team and not daytrading