Well it might have been a poor choice of words, but there was also a time where online advertising was a mayhem with no centralized inventory - it just evolved

Well it might have been a poor choice of words, but there was also a time where online advertising was a mayhem with no centralized inventory - it just evolved very quickly. Well before online ads and a DIY (or in house) media buying for small companies, there used to be agencies that would help with this - media agencies exist for quite some time... but outside of this, I guess it would depend on the product/service. Basically they had centrals to buy media space a "re-sell" it. Other then that, you had to go POS (Point of Sale) communication, or go to someone who bought and managed media (agencies).