As beachwanderer already said, there was substantial demand for the book even before the Third Reich. It seems that over 240,000 copies were sold until January 1933, which isn't exactly shabby. And the claim that no one read the book doesn't seem to be true, since demand in libraries also seems to have been high before 1933.
https://de.wikipedia.org/wiki/Mein_K...marer_Republik
https://en.wikipedia.org/wiki/Mein_K..._and_royalties
Of course, during the Third Reich the real money grab started. Cities were forced to buy the book as a wedding present to newly married couples (though it seems Frankfurt/Main and Leipzig successfully refused to do so) and , very interestingly, it was made illegal to sell second-hand copies.