What does, "you're buying a sorrow that can't talk" mean?

Difficult times.

This line is taken from a part of the novel where Pa Joad is at a distress sale, which is a type of sale where people sell things (like the name suggests) when they are in distress and are in dire need of money. The context of this line follows: "you are buying years of work, toil in the sun; you’re buying a sorrow that can’t talk." With personification, it is communicated that what is being purchased represents difficult times.

"You’re buying sorrow that can’t talk" means that the seller is experiencing hardships and that selling his possessions is similar to selling his life.

In the book’s context, it depicts the bitterness of the farmers towards the people who are purchasing his possessions at such a cheap price - lower than how much he had bought them for - in a time when the people should have been showing their support to each other.