In this context, "selling" can't just mean "exchanging for money", it has to mean something like "exchanging for a valuable consideration." The valuable consideration that you provide in exchange for using Google is your attention on their ads, and your behavioral and personal data that can be sold.
And I would say that Google can fairly be called a unique product in the market. They are the default search engine on almost all browsers and operating systems, their mobile phone operating system owns 70% of the global market share, and most of these devices give Google's search engine preferential treatment, their name is synonymous with searching on the web, their market share is over 90%, they have more data on their users than probably any other company and can provide more personalized search results than anyone else, their web index is clearly more complete than e.g. Bing's (if you do a domain-restricted search, Google often finds twice as many results as Bing).
It's true that alternatives to Google exist, but Google's overwhelming market dominance makes it imo difficult to argue that they aren't a monopoly in practice.