Taxes are too low...
Since no evidence or reasoning is offered, I am not sure how you came to this conclusion.
Have you ever seen team owner that make money on reselling?
Please do show me a team like this. How much money do you think they have earned trading? How many teams like this do you think there are in BB?
And is it right that players are buying with the aim of being selled to other team? I don't think so.
Actually, as long as it is not possible to make extremely large profits, this type of trading is an expected feature of the transfer market. So you're wrong.
And I think that such kind of trades must be prohibitten. It's normal to train players and then sell them, or buy player to increase team level.
So how much is normal? Is training 1 skill level normal? Or maybe 2? Or is it normal to spend 5 season training each player before you sell them?
Now taxes take into account two parameters: transfers' frequency and number of days playes spent with a team. I think that the second parameter must be replaced by a number of minutes player spent on the floor at current team.
As someone pointed out, the system isn't broken, so there is no reason to fix it. As for your particular suggestion, there is a cost for having a player on your team, his salary, which being paid weekly regardless of the minutes played.
"I don't know half of you half as well as I should like; and I like less than half of you half as well as you deserve."