The greenest way to make more coins for your stuff is to upgrade your dispensary, man! 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. (eg. you can sell Purple Kush 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 would be the Dispensary Fee. Since Floyd buys everyone's unsold items every day, he's added a "Dispensary Fee". The extra Coins added on to sold items lets him continue helping everyone and the farming community.
So go forth my young POTrepreneur pal, and start raking in those coins!