How Can Amazon Vendors Sell Books for Only 1 Cent?

But the price is not necessarily going to be higher on the paperbacks, because the prices are set by weight band, not simply the weight.

This is the discount system I mentioned. Note that one price covers a big weight band, e.g. 251g-500g.

Let’s say the average paperback I’m selling weighs about 300g, and the average hardback weighs 1200g. Even if I sent out one hardback for every five paperbacks, the average weight would still be (300 x 5 + 1200)/6 = 450g, which is still in the same weight band. So all my packages, including the light paperbacks, are still charged at the same price.

I assume that the one-penny sellers adjust their stock so that the vast majority of their sales are lightweight paperbacks, which allows them to sell even giant doorstop volumes without losing money on them.

Other factors that may be at play:

Sales on Amazon (at a penny or otherwise) may not be the entirety of the seller’s business (slipping a sales brochure or leaflet in with the book might make it part of their promotional mailshot).
Not sure if the Amazon T&Cs permit sellers to capture customer addresses for future mailshots, additionally, but if they do, that could be another one.

Postal discounts might be dependent upon volume - therefore it might be worth their while having a load of low-profit consignments going out in order to bulk up the numbers and achieve a price breakpoint with their courier

I don’t know about this. I buy lots of penny books from Amazon, and I’ve never gotten a sales brochure with them. And if I ever got a bulk mailer that could be traced back to an Amazon seller using data collected from sales you can bet I would be complaining to Amazon about it, and not buying from Amazon anymore if they didn’t do anything about it.