I have never heard of a farmer selling or giving away manure, but that might have happened. That was a very useful fertilizer for the farmer.
Usually the manure is a product of large stock yards or race tracks
In the case of Chicago race tracks a lot of manure was shipped on the old PM to mushroom plants in Michigan.
The manure was shipped in gondolas and weighed in New buffalo Mi. If the car was too heavy some had to be unloaded.
All that went away in the 70s as trucks started hauling the manure