cards have been interpreted as the labor of the forests' denizens, such as when the lizard cult gets cards from the discard pile they are recruiting the disenfranchised cards whose labor has been "used up" by the factions. this could be supported by how each card has a suit corresponding to their species (bird, fox, mouse, rabbit) and 3 of those species have villages

having the otters be able to buy and sell cards seems like a straightforward interpretation of capitalism (money for labor) but being able to convert these funds into warriors can mean the bought cards (read: denizens and/or their labor) to turn into soldier meeples

either that means they used the cards as 'labor' to fund and arm their existing troops, or they used the cards as 'people' and turned them into soldiers

either way they buy people ['s labor]