Amazon has more sellers because it is the only* platform.

Most of the book sellers are Half Price Books or Goodwill.

As of 2021, Amazon charged a 15% fee AND a $1.85 fee for every book sale, left you to cover shipping, and you paid for your own warehouse. So on a $10.00 book, fees are already at 33.5% BEFORE shipping and handling.
Oh wait, there's the cost of goods as well. It's not like you're getting books for free (unless you're Goodwill). Publishers usually charge stores about 40-60% of the cover price.

So my ten dollar book gets me 6.65 before shipping and handling, 2.65 after shipping, 2.55 after storage if I have a great deal on a warehouse, and NEGATIVE $1.45 after I pay the publisher.

A $30.00 book will only net me $7.52 after that. From that money, I need to pay employees and invest in improvements to my store.
Bookshop just hands my store $9.00 on that same sale, and they store and they ship.

*I know Amazon isn't the only platform, but they've dominated to market to the point of monopoly.

> I just can't quite figure out what Bookshop offers that your local bookshop does not offer

In short, Bookshop is a tool that good local shops can use to boost their chances of making a sale.

Bookshop is a great resource for indie stores that focus on promoting local authors and therefore have less in-store space for James Pattersons or Colleen Hoovers. If a fan of those books passes through town, however, the bookstore can still make the sale and gain a customer through bookshop.

It's great for niche bookstores. I visited a small bookstore in Utah once that focused on mining history, Terry Tempest Williams desert ecology stuff, and touristy coffee table books of Utah scenery. If Bookshop had been a thing then, I could have asked for a copy of A Man Called ove, and they could have said "Sorry, we don't carry that one, since it's not Utah related, but you can order a copy here," and hand me a card for bookshop.

It's also a lifesaver for used bookstores. They tend to have little control over the books they have. Of course, they decide what to keep and what to put on the shelves, but they can't just magically make Brandon Sanderson books show up. At best, they can have some kind of request system that will flag books and send a notice to a customer. Being able to still sell a book without setting up the entire enterprise of a new bookstore is a saving grace.

I worked as the store manager of an indie shop for a few years after running their online store by myself.

Abe hardly benefits the actual book shops. It's crowded with megasellers, just like Amazon is. I would say half of listings on both sites are by HalfPriceBooks or Goodwill. Most sellers who move over 1,000 books a year online will use an automatic listing service like Neatoscan that posts on Amazon and Abe (and like 10 other sites) at the same time.

If you want to benefit actual book stores, buy from their website, call them and buy over the phone, or go visit them in person. Bookshop gets an honorable mention, because they share 30% of the profits with the book store while hosting the site and storing the books and doing the shipping. It's incredibly generous.

>Depending on the author they may be able to put pressure in the publisher

Like Brandon Sanderson is trying to do with Audible/Amazon