So the card is declined, which is fine, I'll re-enter it! But the book I ordered doesn't go back into inventory right away. And now, hours later, it's still not in inventory. I kind of wonder if maybe they wait for the seller to put it back in inventory!
Now, in this case, that doesn't cause any problems; no-one else in Canada has the book for that cheap so I'll just wait. But if I was buying a book that was in good supply, I wouldn't wait -- I'd just buy it from the next seller in the list. So the net effect of their inventory management is that a card declined for technical reasons leads to a lost sale for the bookseller the buyer happened to choose, even though the buyer still gets the book.
I don't see why they'd just give a second chance at entering credit card info for the same book.