Let's look at stocks next. What is stock?
This is a global online resource where customers from all over the world can purchase completely different illustrations (and not only) at an affordable price. The price for an illustration or any other product will not be very high due to the fact that the number of purchases for one product is not limited. This means that if you made a popular and sought-after product, it will be bought many times and in relation to a more expensive product that is bought less often, the first one will win.
However, stock is a fairly general concept for all the sites listed below, most of them do not identify themselves as such. But the sales mechanism is similar for all of them.