In a post of Super Paper Mario Wii Cheats one user, Overlord Goomba, posted a tip for making a lot of coins in a relatively short amount of time. The trick involves using a Catch Card SP to capture an Amazy Dayzee and then sell it at the card shop. Catch Card SPs usually cost 100 coins in the Flopside card shop and the Amazy Dayzee card can be sold in either card shop for 300 coins. This trick can be used to quickly max out one’s coins (and still have more cards in reserve).
Unfortunately, this trick requires access to level 5-2 and Flopside, so it’s not something that can be done early in the game.
Once you have at least one catch card (I’d recommend having two for the first trip) you’ll need to head to 5-2 and move through the star doors until you reach the door with the Yoshi statue. Just after the gap with the floating platform, but before the Yoshi statue, you’ll find an Amazy Dayzee in 3-D mode. As soon as you flip, just use the catch card.