I don't like it that you got a pearl with a chunk missing at the drill hole! They are not imitations, that is the mother of pearl bead that gives it the bulk of its shape. They put those beads into a tiny slit in the oyster with a piece of nacre producing mantle where it forms a sac and coats the bead. Blemishes or dinks or divets are common in Tahitians and you pay A LOT more to get ones that aren't blemished. The broken chunk is not good, but overall it has fantastic color! The divets just prove it was grown in a shell, not a factory!
I would keep it and if restrung, put the missing chunk one at the back, maybe even at the clasp, if it is not too big for that position. Personally, I would not continue to buy from that seller- he has no eye for detail to let that broken one go by.