The manufacturing in Marx time was still mostly manu (manual) job.
Capitalists build facilities, set up factories, bought parts and hired workers. Then sold the products and got paid. It could look like the workers did all the job.
Marx totally discarded the the value of invention, management, organization, innovation and commerce.
The inventors, designers, managers, sales and marketing were just parasites, totally useless for generating any value. They lived by exploiting the sweat of the poor, exploited workers.
He did not missed these people, he actually argued that they are just useless exploiting parasites, skimming the real workers.
The USA now in Marxist view is actually some kind of global capitalist, exploiting poor workers in China. It must be brought down!
In Marx view, the purchasers and the sale people were just in business of cheating the suppliers and the customers. The costs of facilities, parts, management, engineering etc, were just some constant and variable costs to be passed to customers and the !Only! real value created was done by the workers.
It was always wrong to thing that way, but in Marx time, the workers were actually bigger part of the business than they are now, so it could look little more plausible then.