I just started Hay Day, am level 10 and make a lot of money in my roadside shop with this tactic: Always put things on sale by lowering the price. More people will buy and it will give you more money,.
Let's say you put a nail or whatnot on sale for 200 coin
Not that many people will buy. But if you lower the price to 50 coins, chances are you will have more people buying.