If a farmer can't buy the amount of grain he produces is because he's not producing all of it. Why?
Someone built the machines that the farmer used to farm. Whoever built those machines did partially produce the grain. Same goes for whoever owns the land and whoever produced the seeds and whoever marketed and sold the grain.
If there is a farmer that:
- Built his own farming equipment with raw materials mined by himself.
- Owns the land the wheat is growing on.
- Produced the seeds to plant the wheat.
- Marketed and sold the grain.
Then that farmer produced that grain with only his labor, therefore he should in theory be able to buy the same amount of grain he sold, just make sure to put the right price on it. But what is the point? If he did all that labor himself to buy some grain, he could've just kept some grain for himself.
A baker that bakes a loaf of bread didn't produce a loaf of bread. It turned some labor+materials+capital into a loaf of bread. If those materials and capital are not the fruit of your labour, neither is the entire loaf.