Solution for sharing of card in Steem Monsters.
Each card that is played in a season is registered to the account that uses it 
and is disqualified from participation by any other account for that season.
Easy enough solution.
Please enjoy the Wise Owl picture.
This would have issues if someone used a card (say just for their daily or something), then sold it to someone else though
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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.
Thank you so much for your comment and interest in helping this "Game" progress.
Just saw your comment on my post. The solution you propose sounds good to me, especially in the short term. The way things are now, I'd support the change being implemented as soon as possible and before the season ends. People are under the impression they have 6 days before any action is taken and those with high SP can use their resource credits to create accounts for free and use Steem they earn in interest to buy the starter packs. It only takes a couple of hours of automated play to get past the 30 pack barrier. A no brainer for some.
Thanks for your studied reply.
It's a simple and quick solution . . . Like Barney Fife says . . . "Nip it in the bud!"