The buying part is easy
The buying part is easy because Amazon makes it so. Nearly every potential reader for your Kindle books already has an Amazon account and a Kindle device. Once the reader is on the website, it’s easy to buy: one click. It applies for the other online stores too, more or less.
To get your reader there is the hard part. There’s nothing easy in addressing numerous potential buyers. Readers aren’t just readers. They are readers of romance, of science fiction, and a thousand other genre or sub-genre. There is no strength in numbers here.
How many listen to your call-to-action? How many land on your book’s page because others recommended it? How many follow the yellow bricks road of paid ads?
Sales and marketing are skills writers need nowadays. The cost of online ads is easy measurable. You will have to do some ad buying in order to have readers do some book buying. It might be that it’s worth it.