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.