As mawit already told you staking SPS is an option.
You can buy spliterland cards and put them in rental and it will give you rental income in DEC and you can sell card when it appreciate.
I will suggest you to keep an eye on "Chaos legion" sale(To be announced.).(Staking SPS is good suggestion as I learned user need to have staked SPS to take part in that sale.)
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Are Splinterland cards the same as Spellbook of $10?
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Spellbook is required if you want to earn DEC(in game currency) as well as season rewards. Spellbook contain level 1 phantom cards(that cannot be sold/transferred.)
I am not sure if SPS staking require spellbook but I do not think so.
Currently I will suggest you to keep an eye for SL presale of chaos legion, that probably will happen on September.
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