I have a suggestion that is linked to what is done in real estate market : additionnal taxes calculated on the increase in value.
Buy a player for 100k. Sell him for 300k. The additionnal taxes will go on the 200k of difference. Higher the increase in value is, higher the taxes % is .
This wont prevent noobs from buying too expensive players (like this one in my division did few days ago http://www.buzzerbeater.com/team/84853/transferhistory.aspx ), but it will make day traders look for the right price to re-sell (in a short term trading).
I do agree with managers who want to keep daytrading as a part of the game. But a "honest" 200% increase in value cannot be reached in 2-3 weeks. Even with the best trainer and elastic effect in training etc.
A 10%-50% increase in the price (on a 2 week term) is still reasonnable and can be linked to the demand/offer evaluation (a player bought for 100k can be selled 150k the next week, this should not be seen as cheating).
But a 200% (or even more) increase in value in a 2 week term is not a "honest" offer.
A "honest" and strong increase in value can only exist if the player has been trained well for a season time (and get his 5 or 6 ups). Day trading should exist, but not with that huge sales margins.