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That is a good idea.
I could collect more DEC (Dark Energy Crystals) (and maybe even reward cards) without having (owning) any card.
But card borrowing would be much easier with an internal (in-game) card delegation/lease market, but currently there is no internal (in-game) card delegation/lease market in Steem Monsters/Splinterlands.

The lease market is through peakmonsters.com. That's where I do all the leasing, etc. It's a project by the team behind @steempeak, but I believe they collaborated with @steemmonsters for the project, too.

The website is actually a well-developed marketplace for Steem Monsters/Splinterlands cards. You can buy cards in bulk, bid on cards for sale, and lease. They also have a pretty good block explorer for all steem monsters activities. You should check it out if you decide to go back into the game, too.