A different solution for elimintating the day-traders
I don't know much teams who sell the same player after few days for much more money.
Only when a player is getting better (skills), it should affect his price.
So...
1) In case player's skills had not been changed between buying and selling, the difference will go to waste (the day-trader should not get this money. You can say that it will go to the players great agent).
To give a solution for players that are being traded after a short period and just for great and easy money revenue, I will add this;
2) Have a formula that will allow a growing value for a player that had been trained more;
Examples:
1) A player that had been bought and sold w/o training cannot be sold for more than his original price
2) A player that had been trained for 1 week (for more than 48 minutes per week of training) can sell for the buying price + X*1.
3) A player that had been trained for 2 week (for more than 48 minutes per week of training) can sell for the buying price + X*1.1.
4) A player that had been trained for 3 week (for more than 48 minutes per week of training) can sell for the buying price + X*1.15.
Meaning that unlike now that the price is only calculated upon owning a player, it will also calculate the revenue upon training a player down, and maybe using that player (on real-games!!! not scrimmages or PL).
3) To eliminate those repeatedly last seconds callers, I already suggested the auto-auction suggestion that will also improve the competativenss of this game. But, there are other things that can be done on that matter:
a) A user cannot bid on a player while there is another player that he had issued a bidding on that player current auction.
b) A user can bid a maximum of three times on a specific auction.