[ksec]

It is not a clear cut 50%, but it is about options. There are plenty of alternative for books, ebooks etc.

In a monopoly, a single seller controls or dominates the supply of goods and services.

The term you may be looking for is monopsony.

In a monopsony, a single buyer controls or dominates the demand for goods and services.

[libertine]

But isn't all dependent on the frame?

Amazon owns the monopoly of the ebooks for the KPF (Kindle Package Format), but it's not the monopoly of ebooks market.

Google has the monopoly of search, but doesn't have the monopoly of advertising.