Then another copy came yesterday.
At first I thought I'd absent-mindedly ordered two copies, since I buy a lot of books, and this one, though rare, was not expensive (around $20, including shipping). The book had a single small print run which bankrupted its publisher, and there's no significant demand for it, so it's hard to track down. The first copy I received was in pretty poor condition, and it seemed fair that I might have decided to sacrifice quality in favor of actually getting my hands on a copy.
Anyway, the extra purchase was bothering me this morning, so I decided to track down the order confirmation, and... there isn't one. I threw away the envelope in which the first copy arrived, so I can't be absolutely sure that they came from the same place, but I am pretty-close-to-absolutely sure. Looking at websites where I might have purchased the book, it's clear that the only copy I ordered is the one I received yesterday, which was listed as being in "very good" condition. Perhaps the seller realized too late that they'd sent out the wrong copy, and sent out the good copy instead? I don't know.
I suppose the Right Thing to Do would be to contact the seller, but I think I'll give it a few days to see if they contact me first.