How to get money VERY quickly
To get money extremely quickly in Hay Day, first, go hunting through your newspaper, and other people's newspapers, for construction equipment and saws, dynamite, and other goods selling in buld (I.E. 5 for 400 or something). Buy them all. Then, turn around and sell them individually for between 180 and 220 coins apiece, and put an advertisement on the very last one you list, so that when people go to the store, they will see all of them.
Diamond Trade Swap
Look for a farm called 10=1 diamond run by kyleigh.. I'm not sure how it works but every 100 you spend you get 1 free diamond... I'm currently on 20 diamonds.
Getting lots of extra items, tape, and more
Go to a friends farm and click on the toolbox. Sometimes it locked just click out of it but the reload the friends farm and find and click on it again. You can do this over and over again. When you reach your max u will continue to get the same item you last received. Hope this helps.
Provided you have an available slot you can sell stuff at the roadside shop as often as you want. You can only get a free advertiment in the newspaper every 30 mins.
These can either be found in the treasure chests or you can buy them with your diamonds. A good place to find them is when your are havesting crops as they will just appear and go into your barn.
Buying Missing Crops
If you are missing some crops simply go to a friends farm and find their roadside shop and tap on it and see what they are selling. Or if your level 7+ open your newspaper and see whats being sold.
Buy a lot of wheat until silo full plant half of it then turn your device up right and shake it while cutting down wheat u will get free items.
Open your mine and use TNT. It will produce diamonds every couple of blasts. When you have enough diamonds, pay for Tom's service and have him find nothing but TNT and load up on it. Then blow the mine and use as much metal as you can, again more diamonds will come every couple of blasts. Sell what metal you can't store. But you have to use tom and get a bunch of TNT.