Plant pumpkins and sell them when they're fully grown. The closer it is to fully grown the more coins you get selling it. You can only carry one pumpkin to the stall at a time, so plan when you plant your seeds to make the most money. 

WASD to move.

E to pick pumpkins and to sell them when standing at the stall.

Q to plant pumpkins.

To buy more seeds go to the store menu.