To sell items in Hay Day simply visit the roadside shop by tapping on the market stall at the end of your driveway. Tap on an empty box to bring up the item selection menu. Once you have chosen the item you would like to sell, simply tap on the + or – symbols next to the item to adjust the price. Once you are set, tap the green tick button to make the sale!
Protip: If you press and hold on the + or – the price will increase faster!