One of the factors that influences the development of your farms is the machines you have (strictly speaking - selling the old vehicles and buying new ones). This can be done in the farming shop [P].
To buy a new object, you just have to select the proper category and the vehicle or implement that you're interested in. If you only have enough money, it will be automatically taken out from your account and the machine you've purchased will appear near the shop. Then, you can go to pick it up with a proper vehicle or simply reset it, to find it on your farm. You can find the reset option in your PDA [I], in the vehicles tab. Select the machine and choose Reset vehicle/tool.
Sometimes you will have the chance to buy something at lower costs. You can check the current occasions in the sales tab. When something new appears in the tab, you will see its price and a comparison to normal price of the product. If you happen to find the machine that you currently need, do not hesitate too much. Paying 10% less for an expensive machine can save you a lot of money.
Selling machines is also not so easy. The simplest way to do that is to enter the farming shop menu, choose the appropriate machine in the Garage tab and select: Sell. However, this won't bring you much profit. It is far better to transport the machine yourself, all the way to the shop, as you will then receive 20% more money than when selling it automatically, through the menu. Go to the store and place the machine on the spot that you can see in the picture above. Then, move to the icon nearby and select the appropriate option. Doing it this way, you will earn some more on every selling transaction you make.