While I am not overly concerned about the TL issues and day trading, I see this as a huge topic of discussion so I offer 3 ideas here. I apologize if these have been suggested and I have yet to see them.
1) An owner could be charged for placing a player on the TL. Perhaps this charge is based on their salary or based upon some formula that takes into account (skills, age, potential).
2) Every team gets 3 buys and 3 sells a week, or maybe even 4 total TL moves a week. If a player goes over that "soft cap" they are taxed and the tax increases for each move beyond that.
3) A purchased player must remain on a roster for a minimum of 3 weeks before they can be placed on the TL.
The numbers that I chose in each situation are arbitrary and could be altered as needed.