First, I would ask the seller what he means by "copal amber". Does he think the beads are a natural material or a man-made resin? If he says they are man made, then I believe you have your answer, since it would be impossible for him to figure out exactly what plastic it is. It may even be a mixture of more than one plastic.

If he says a natural material, then I would ask which one and see what he says. If he says copal, then you could suggest a hot needle test (plunge a sewing needle heated in a flame into the side of the hole) but he may not want to damage the bead. And you will have to believe his result, which would be a stinky plastic smell for plastic and a pleasant piney odor for copal (or amber).

Acetone will soften many plastics (but not all, and some will only soften very slowly) and will also soften copal so that would not be a definitive test. But, we know from the pictures they are not copal.

Please let us know what the seller says in any case!