The Buy window is important for getting started on an empty farm. If you are raising crops, you'll need to use it to buy machinery to tend your fields. If you are raising livestock, you purchase your initial livestock here.
It can be opened from the Windows dropdown, or by clicking 
in the top toolbar.
To purchase something, select the appropriate category, find the item using the Up and Down buttons, then click the BUY button. You'll then be dropped to the edit window where you can place as many of the item as you want by clicking on appropriate tiles. All purchased items must be placed on your property. Additional placement requirements and exceptions are detailed in the subsections.
To cancel/stop the purchase tool, you must select another tool.
Machines[edit]
Machines are used for tending your fields. The crop duster is automatically placed in an airport when purchased. If there is no room in an airport, you won't be allowed to buy the crop duster.
Other machines can be placed on a clear tile, a road, or inside a shed or barn. Specific placement inside the shed or barn is managed by the game.
Machines will be automatically leased if the AutoLease option is on.
Machines can be sold for full price if in Good (A) condition, or for a reduced price in worse condition.
Seeds[edit]
Seeds for crops will be consumed when a field is planted with the matching crop. They can only be placed inside structures, specifically sheds, barns, and silos. After being purchased, they can also be moved into clear tiles that you own with the Move Object tool. Specific placement inside the shed or barn is managed by the game.
Seeds will be automatically purchased for the current price if the AutoBuy option is on.
Seeds cannot be sold. If outdoors, they cannot be bulldozed. To discard seeds, you must bulldoze a structure that is storing them.
Livestock[edit]
Livestock can be placed on clear tiles that you own or on roads.
Make sure to place your Livestock inside an enclosed pen. You can still sell animals that escape, but they won't have access to food or water.
Livestock can be sold.
Structures[edit]
Water pumps can be placed on any clear tile on the map, but will only function beside a water tile. Other structures that you purchase from this window can only be placed on clear tiles that you own.
Structures cannot be sold, they must be demolished.
Sprays[edit]
Sprays will be consumed when a field is sprayed with the matching chemical. They can only be placed inside structures, specifically sheds and barns. After being purchased, they can also be moved into clear tiles that you own with the Move Object tool.
Sprays will be automatically purchased for the current price if the AutoBuy option is on.