Only sell back again items as a last resort to acquire coins or if you're sure you don't need it as you will only receive a quarter of the prize you payed. If you used gems to acquire an item you will not get gems back again when you selling it.
Always pay a visit to your neighbors each day and help them out to earn coins and XP that will level you up faster and unlock more items in your case to get with the coins you gained. You can also use this strategy when you are close to leveling up and don't have any energy, so go to your Social option at your main-menu and help your Neighbors. Performing this could earn you more XP points and raise your energy meter.