I think that's a universal argument for making all farms worthless, outside their subsistence value. All the produce must be sold; your argument means you can always argue that the value comes from the selling, not the farm.
I am simply making a statement that differences in quantity become differences in kind. You can separate things by revenue generation.
If you buy from 1 farmer and then sell wholesale to 1 customer your not going to make significant money. If you buy from 1 farmer and then sell at a stall to 1,000 people you can make meaningful profit.
