The phrase "to buy a button" means to spend money on something that is completely useless or unnecessary. It implies wasting money on something insignificant or trivial. Full definition
The platform allows businesses to embed buy buttons anywhere online they want their products to be purchased — from social media to apps and email promotions. (entrepreneur.com)
I first bought a button down top in this pattern and fell so in love, when I saw it came in a dress too, I had to have it. (strungingold.com)