One of the critical faults that led to the Great Depression was overproduction. This was not just a problem in industrial manufacturing, but also an agricultural issue. The resulting glut leads to lower prices and possibly unsold goods.
Margin
In 1920s American farmers were producing far more food than the population was consuming. As farmers expanded their production to aid the war effort during WWI they also mechanised their techniques putting farmers into debt.
Banking
The main cause of the Wall Street crash of 1929 was the long period of speculation that preceded it, during which millions of people invested their savings or borrowed money to buy stocks, pushing prices to unsustainable levels.
International Trade
The concept of buying on margin allowed ordinary people with little financial acumen to borrow money from their stockbroker. This meant companies had to purge their supplies at a loss, and share prices suffered.
Buying on margin
A wave of banking bank runs during which large numbers of anxious people withdrew their deposits in cash, forcing banks to liquidate loans and often leading to bank failure.
Shaky banking
During the Great Depression, America embraced a policy of protectionism, using trade restrictions to try and strengthen American products with legislation like the Smoot-Hawley Tariff Act of 1930.