I'd be a little worried about the fact the seller has 11 negative and 14 neutral comments in the last year. That's very high! Most of the time when people leave neutral comments on eBay, they wanted to leave negative feedback, but didn't want the seller to reciprocate with negative feedback.
You can bet that the seller is only guessing on the nacre thickness. I doubt they deal in such quantity that they regularly cut the pearls (or x-ray them) to determine nacre thickness.
The luster looks good in the picture, and the blemishing is minimal. I would ask the seller if the necklace in the picture is the one they are selling (I'm betting it isn't). Often, an eBay seller will use the same picture over and over. This seller has done this before. Looking at their past auctions, most of the auctions they list as private (which worries me), but of the few that are public, I see at least one picture which was used for more than one auction.