Each company takes pains to pick out any rare books that might be difficult to scan automatically. The systems rely on barcodes and International Standard Book Numbers (ISBNs), relatively recent innovations in the history of book publishing. Thriftbooks has a special “vintage” team dedicated to picking out rare, older books, trained to recognized first editions. Discover Books relies on its regular scanning system to pick out potentially valuable vintage books.

From that point on, it’s all about software. Say a new copy of “A Visit From the Goon Squad” is scanned into the database. The software races to figure out how many copies are in stock, how many copies have been sold, how the price has changed over time, what the current average, high and low prices are on Amazon, Barnes & Noble, Half.com, AbeBooks, eBay and Alibris, and decides: Do we want this book? If the software’s algorithm decides that this is a book that can be sold and is worth selling, it will be stocked and automatically listed in the online marketplace where it has the best chance to be sold. This all happens tens of thousands of times per day.

This is a game of pennies and lightning-quick readjustment. Buyers have no particular loyalty to any of these sellers; it’s all about what’s cheapest and what’s listed first. Each company, seeking an edge, builds and zealously guards its own software. Ward was the lead developer on Thriftbooks’ software before he became president. He has 12 developers, a full-time data scientist and two financial analysts on his staff. Discover Books’ software is known in-house as Trim2. “We have software that we’ve spent years and a lot of money on,” Hincy says, “tweaking to be as optimal as possible to give that book the best opportunity to be sold.”

I asked Ward to track a few books for me, and within seconds he had generated a chart showing their prices over time. For recent books, supply is low, because the book hasn’t been on the market that long, and demand is generally high. Accordingly, Thriftbooks doesn’t sell many copies of new, hot books like “The Life-Changing Magic of Tidying Up,” by Marie Kondo, and when it does, they may go for as much as $15. Donna Tartt’s “The Goldfinch,” a literary hit from 2013, is in its second stage of life; its price, says Ward, has stabilized at around $5. But that book sold over three million copies (including eBooks), which means more and more of them will find their way to Goodwills and thrift stores — and, eventually, to Ward’s warehouses. And so the price will sink lower and lower, eventually bottoming out at a penny.

A “penny book” is something of a misnomer. Used books sold on Amazon typically carry a $3.99 shipping fee. But that isn’t a reflection of the actual cost of shipping them — it’s a function of the company’s rules, which mandate a consistent shipping cost for every category of the product in the Marketplace. Amazon takes a standard cut of every book sold — $1.35 — which leaves each of the penny sellers of “A Visit From the Goon Squad” with a whopping $2.65 to cover the cost of the item, shipping and handling, labor, rent on warehouses and all the other costs that come up along the way.

The sellers wouldn’t tell me exactly how much profit they make on penny books. Shipping costs vary depending on the kinds of deals you can cut with delivery services. “We make more than it costs us on the postage to ship it, but not much,” Ward says. “A couple of cents, to be honest.” But the sellers aren’t selling only penny books — Ward says that less than half of his stock sells for that price. And because processing costs don’t increase with book price, while Thriftbooks may only make a few cents on a penny book, it will make $2, plus a few cents, from a book priced at $2. Not bad, when you sell 12 million books a year. (For what it’s worth, books like “A Visit From the Goon Squad” are usually sold wholesale to Barnes & Noble at somewhere between 40 percent and 50 percent off the list price. List price for the paperback version is $15.95, and Barnes & Noble currently sells it for $9.49, giving it a profit of ... very, very little.)

If the penny booksellers haven’t noticeably cut into the publishing industry’s margins, their arrival has been more ominous for traditional used-book retailers. “It’s really a race to the bottom at this point,” says Carson Moss, the store buyer at the Strand, the 88-year-old Manhattan book emporium. “Everybody ends up fighting over 5 cents, 10 cents, a penny.” Moss says the penny booksellers are an occasional topic of conversation at the Strand, which also sells books on Amazon and elsewhere online. “I would say it has affected our ability to sell as many used books online,” he says. The penny booksellers are sometimes seen as forcing books into a new, disturbingly quick life cycle in which prices drop from full price to a few dollars in months, and then further to a penny in just a couple of years.