The Sell window is the most direct way to generate income. It is the primary means of selling harvested crops, and the only means of selling livestock.
To sell something, select the appropriate category. The window will list everything you own in that category. Use the scrollbar to find the item you want to sell, then click it (make sure it's bordered yellow), then hit the SELL button. You cannot undo a sale.
The sell list will show an icon representing each item, a quality badge (higher quality always sells for more than lower quality of the same item), and the current selling price of that item.
Underneath the list is a counter of the number of items in the current list, and which one is currently selected.
Price fluctuations will be summarized in the subsections.
Crops
Crops will be stored in silos when they are harvested (unless silos are full or you have sold futures).
The current market value of a crop is a combination of the crop's "base price" and the current market fluctuation. The price of a harvest is further modified by its maturity and quality, with each being a continuous scale. In other words, there may be slight differences in quality between two crops that are both labelled as Good (A) that result in slight price differences.
A harvested crop's quality gradually declines while stored in a silo, but over a short term, market fluctuations will probably overwhelm any quality-based price drops.
Bad crops are worth $0.
It's wise to pause the game before selling crops to avoid sudden market value drops right as you're clicking the SELL button.
Machines
Machines are used for tending your fields. Machines can be sold for their full purchase price if in Good (A) condition, or for a reduced price in worse condition (usually half price for each step down).
The value of machines does not fluctuate aside from the quality adjustments.
Livestock
Livestock value is primarily dependent on quality, then age. If an animal is less than Good, it will always be worth less than its purchase price.
An animal that is Good (A) will be worth at least as much as its purchase price, and its value will gradually increase with age.