What's the use of stacking the mainland market?

You can sell cards to get gold coins. In the game Stacking Continents, the market is one of the main sources of gold coins. You can sell things in the market, stack the cards that need to be sold in the market, and you can get gold coins when the progress bar is completed. Moreover, the market has doubled, and 1 gold coin stones can earn 2 gold coins when sold in the market.