If you’re buying them with the intent to USE them to thwart God’s plan for marriage then yes, it’s a sin. I’m not sure why you’d buy them if you didn’t intend to use them.
It’s a hard teaching with all of the pressures of the modern world. I suggest jumping in to the Church’s teaching on the subject. It was eye opening for me and now that I’ve educated myself I fully support this teaching.
GB,
M