The greenest way to make more Coins is to upgrade your Dispensary, dude! With every level you gain a multiplier! Wicked, right?
The multiplier changes the top end price at which you're allowed to list the item. (For example, you can sell Chocolope for 2x the base price!) You'll always receive the amount of Coins you listed the item for in your Dispensary.
If you see others selling items for a higher price than yours, or friends saying your prices are higher than you claim them to be, this is a fee to balance the economy. Since Shorty buys everyone's unsold items every day, she's added a "Dispensary Fee". The extra Coins added on to sold items lets Shorty to continue helping everyone and the farming community.
So go forth my young POTrepreneur pal, and start raking in those coins!