Help Neighbors Strategy
Always pay a visit to your neighbors everyday and help them out as this tends to earn you coins and XP that will level you up more quickly and unlock more items in your case to get with the coins you gained. You can also use this strategy when you are near 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 generate you more XP points and raise your energy meter.
Offering Items
Only sell back again items as a previous resort to obtain coins or if you definitely don't need it, because you will only receive a quarter of the prize you payed. If you used gems to obtain an item you will not get gems back again when you selling it.