Buy in Bulk:
One of the best way to earn Coins in Hay Day is to use the newspaper to buy in bulk. Look through your own newspaper, as well as those of other people, to find construction equipment and other goods, like saws and dynamite. You want items selling in bulk, such as 5 items for 400 Coins. You can then sell each one individually, making a greater number of Coins once all have been sold. Place an advert for the last item that you list - this will mean people will see all the items when they visit the store.
Sell, Sell Sell!:
You should always try to sell any excess items that you have in your Barn or Silo. Not only does this clear room in your storage for future produce, but it nets you a heck of a lot of Coins. Before you go out or go to bed, try to fill every slot in your Roadside Shop. Putting items you have produced from the production buildings in there is best, as they are rarer, rather than items like Eggs and Wheat which everyone's farms are churning out by the dozen.
Some players have one main account, which they focus all their efforts into, and other, helper accounts, which they use to support that main account. Things like buying and selling items at favorable prices, helping with Trees and Bushes, and so on. If you think you have the willpower to manage multiple accounts at once, it's a good way to get ahead at Hay Day.