In my magnanimous opinion, it solves the bigger issue. The buyer would have to wait until the end of the season to use a new card that was previously used in competitions.
Frankly, to sell a card without such notice is just bad form, and bad players would quickly be identified.
I'm sure that our Devs could easily code a non-sell period for a card if this were to be a problem of Titanic proportions.

For now, people who use maxed cards to win champion level prizes and then transfer them to another account to win again and again is a problem that will end this game.