I wouldn't mind seeing some sort of consequence for “fishing” that's constantly going on in the market. Though, I'd rather there be a logical downside, instead of BB's just restricting it altogether.
My suggestion would be to implement:
1. Merchandise drop
When a player is put on a TL, fans will not buy that player's merch on the next financial update. This simply makes sense, cause why would they continue to buy merch of a player that is supposed to leave the club? After a week passes where the player is no longer on the TL, the fans will start to buy his merch again, but it will take 3-4 weeks before getting to 100% again for that player, as it will take time to gain trust of the fans again that the player won't leave the club as soon as they buy his jersey.
Perhaps, level of PR manager could help reduce the impact of merch drop, so level 7 PR would be able to pay the newspapers to write a convincing article that it was just a rumor, and that the player won't leave the club, so the higher PR level, the lesser effect it will have on the merch, although even level 7 PR could only lessen the impact, not erase it completely.
This won't have as big of an effect on lower league managers where the merch isn't high, but could impact top managers quite a bit, especially when they wanna sell their entire team by “fishing”.
2. GS drop
When a player is put on a TL, his morale will drop and it will be harder for him to keep his GS level high. For example, if morale of a player is at level 100 as default, and the player spends 3 days on a TL, his morale will drop to 97 (100-3), so his GS will be at 97% of what it would be if he wasn't put on a TL.
So let's say a player spends 9 days in a row on a TL, and doesn't get sold. His morale would be at 91, and his GS would be 91% of what it would normally be. So if he would normally have GS 9, he would now have GS at 8.1 (91% of 9).
After the GS update, player's morale would return for 1% after every 2 days that he doesn't spend on a TL. So in examples:
→ Player is put on a TL for 3 days → His morale immediately drops from 100 to 97, and stays at 97 until next Friday (GS update) → Starting on Friday, his morale will start to get back up if he is no longer on a TL, and will be back to 100 until next GS update.
→ Player is put on a TL for 4 days → His morale immediately drops from 100 to 96, and stays at 96 until next Friday (GS update) → Starting on Friday, his morale will start to get back up, but let's say he's put again on a TL on Monday → His morale would have risen from Friday till Monday for 1.5, and would now be at 97.5 → he's put on a TL for 3 days now → Next Friday, his morale will be 94.5.
♦ When a player changes club, his morale is back to 100.
♦ Player's morale only impacts his GS and nothing else.
♦ Player's morale would be hidden for the managers, and not visible.
♦ A bit of randomness could have an effect too. For instance, aggressiveness of a player could play an effect on how much their morale will drop. Aggressive players may stay fired up even when they're on a TL, and their morale wouldn't drop as much, while non-aggressive players would take it harder, and their morale would drop more.