I think that we should have to wait some period of time, say 30 days, after signing a new player before he can be placed again on the transfer market.
This will surely end transfer market "day trading" (an annoying an unrealistic aspect of GKO) and will force managers to allow a player to adapt at least somewhat to his new team before he can be re-sold. Thirty (30) days is a mere 8-9 gameweeks, only about 1/4 of a season. While we don't have transfer windows like the real world (probably a good idea) this would serve their fundamental purpose, imo.