Splinterlands has an interesting economic model. Sometimes it feels like one elaborate pay to play game, while other times, I'm just glad that I can enjoy the game. While it may not be perfect, the game has been around for only a short period, but you can tell that the devs are ambitious. The game is in a constant state of flux, and regardless of how you see it, we're moving.
The best way to earn from the game is through participating in ranked battles and tournaments. The latter is something that I need to get more active with because there are some rich tournaments out there. However, if you'd like to invest in cards or earn from your current deck but don't want to actually play the game, then this is for you.
The best way to sit back and earn passive income with your cards is through the leasing market. I don't know if there are other lease markets around but I use the service through peakmonster.com.
Generally, the procedure to lease cards involves;
Naturally, the cheaper your offer, the better the chance of your card being picked up. It is a very competitive market and it is an alternative to selling your cards if you don't need the bulk sum. So, if you have some cards just laying around in your deck, you might as well put them to use by listing it on the lease market, and then earn passive income through them.