Yet to be published yet still for sale

I have recently published two novels, Time to Talk and A Serious Business. They are both general fiction with elements of humour.

A third, The Ears of a Cat, is published in paperback in January 2020. There is still humour, but more black than before.

But though it is yet to be published, I see three second hand copies for sale. How can this be, I ask myself - not expecting an answer.