Corruption undermines democracy and erodes trust, corruption is more deadly than sickness. Many people associate corruption with the government when we discuss it, forgetting that it begins with me and ends with us. Consider a scenario where a nearby farmer harvests grain from his land and sells it for three times the cost. A manager of a company will have an affair with his worker in exchange for a job, etc. The government is not to blame for any of these..
You know, that is not a suitable example for us to talk about corruption, it is rather an example of speculation. In reality, the prices of the open market would not allow a farmer to sell their products for the triple of the price as easily as you portrait.
A better example for corruption would be a police person abusing of their power for financial gain, that is the simplest example. Though, I would agree with you that corruption does not start and end with big position in government, it also starts with the common people like you and I.
Your example of the speculating farmer reminds me of some countries which went hrough socialism, by the way, in which people who worked the land were forced to sell their crops to the government.