That depends, but ultimately yes, at the moment it's safest to make the assumption that any used card has been mined on.
While I don't believe there is a concensus on whether mining itself is inherently more damaging to a card than general use (e.g. I'd rather have a mined card which has been sat at a constant <65c 24/7 in a well ventilated dust free case than a card used purely for gaming which has been overclocked within an inch of its life and run at 100c+ 7-8 hours a day in a hotbox case with no airflow), unless you know the seller well, unfortunately there's no way of knowing how it has been treated.
While I don't believe there is a concensus on whether mining itself is inherently more damaging to a card than general use (e.g. I'd rather have a mined card which has been sat at a constant <65c 24/7 in a well ventilated dust free case than a card used purely for gaming which has been overclocked within an inch of its life and run at 100c+ 7-8 hours a day in a hotbox case with no airflow), unless you know the seller well, unfortunately there's no way of knowing how it has been treated.