I suspect that if you’re the type of person who might think to torrent a book cover, you’d probably just torrent the whole book.

The implication is that a bookseller would print them so they can sell their "unsold" books.

That's a fair amount of effort, expense (printing), and labor (assembly) just to try to sell a book that you've already determined isn't selling very well!

It might also be fraud where selling a coverless books is just breach of contract? IANAL but would be interested in finding out

I suppose you could do it for books that were selling well, but they might get suspicious if your bookstore was the only one.

You can usually download high res covers from Amazon if you want.

These booksellers could just make copies of the ones they have.

Just like the album cover scrapers of yore (and probably still...)