As the productivity of labour rises in industry the number of
commodities
produced grows year by year. This is true both in terms of the number of
types of commodities and in terms of the number of copies of any given
commodity. One only has to think of how many commodites currently sold
simply did not exist 100 years ago to see how true this is.
Each of these commodities has to be sold, the money for the sale
collected, the wholesaler has to issue invoices etc.
Whilst the value of the individual commodity may fall as a result of
automation, the labour of selling has not been subject to the same
degree of automation. There has obviously been some degree of
automation but productivity in sales has not grown to anything
like the same extent as productivity on the factory floor.
1930s cash registers require only marginally more labour than
current ones, indeed, tests in asia have shown that abaccuses
are more labour efficient than modern cash registers.
The rise in productivity in advertising is probably also quite
small.
If the labour efficiency in the sales sector stagnates whilst
the productive sector turns out an ever increasing flood of goods,
then more and more people must be involved selling them relative
to producing them.