I mainly used the Evil Empire, Amazon, in both cases. Though there are other options, it tends to be where the bulk of buyers are. Best is to use Amazon AND all the other available options.
It's very easy to open a second hand bookstore there or list books on ebay, though harder to sell them! Requirements are a reliable internet connection, access to cheap but good packing materials and a post office, and most of all a keen interest in books.
The second hand bookstore was almost an accident -- or a God thing, depending how you want to look at it. I read a lot, hubby reads a lot, and we have a small house. I started selling books simply to make space. At that time hubby and I had a weekend thing of visiting two local car boot sales on the weekend and I saw books there for pennies that I had recently sold for more, so bought a few books to resell. (Car boot - it's a UK thing, like a yard sale on steroids, where all the people who would have had yard sales takes their stuff to a field and pay the field owner a small fee to sell it there instead, so we got to shop 40 or 50 yard sales in one place).
Then I started looking for other book supplies and made contact with local thrift stores to buy the books that didn't sell in store, which often turned out to include some very good books among the piles of trash. That started when I discovered they'd thrown out a book I'd hesitated over risking a small purchase price on, which had a buyer waiting at 60 GBP! At that time, they trashed good books after two weeks on the shelf! My best buy was a signed first edition Samuel Beckett that sold for over $500 in a pile of "trash" unsaleable books a new-to-me thrift in a town we were visiting was going to throw out next day. When I went to the counter with a stack of books from the shelves, they asked if I wanted to buy the forty boxes of books illegally blocking their fire exit at a very low cost. I bought, with no idea what treasure it contained, and made back 2000 or more times what I paid for those books.
It's more difficult now as a small seller since bigger businesses moved in to the used book market. It's simply not worth the work of selling lower priced books. But if you have an area of specialist interest where you know what books are selling for or a knack for spotting good first editions there's still a side income to be made this way. A local thrift might put a permaculture book in the gardening section with a $3 price tag, when used copies sell for $30 or more. It might not sell in the small-town or suburban thrift, but will find a buyer quickly on Amazon or ebay if it's priced right. For two years, I did bookselling as a full-time home-based business. Then things changed, I needed more income when hubby got sick, so went back to my main profession for a few years. I don't actively sell other author's books at all now, as my own keep me plenty busy enough.
Writing and publishing books - I started writing simply because I loved to read and write, had always written since I first held a pencil and was the kid who maxed out her library card at every possible library, every week. It was a lifelong dream to be published but real life always got in the way. Approaching a big number birthday, I decided to go for it. After a few practice books that will never be published, I started pursuing traditional publication for my fiction and had some serious interest from the publishers I had been targeting. But I felt strongly guided not to go with that and to self-publish instead. I worked for months with an editor, made my own cover, put my first book on Amazon and prayed, then got busy with the second book. I posted a lot to online reader groups, also did some low cost paid ads, gave away a bunch of ebooks, and books started selling. I'm now working on book 28. I'm not a mega bestseller, very much a lower-end-of-the-mid-list author. I'm never going to get rich this way. But I am doing what I love and it earns a decent enough secondary income. It's also something that can be juggled with family responsibilities and homesteading. I suggest someone wanting to publish should choose a niche market they have an interest in rather than trying to compete with the big names. That works for non-fiction as well as fiction.
(In case you are interested, this is me:
https://www.amazon.com/Autumn-Macarthur/e/B00N9KRHKY )
Anyone with access to a computer or even a cellphone can write and publish a book. Ideally, they'll get a good cover design and have the book edited and proofread before launching it! But it's completely possible to publish a quality book at no upfront cost at all. Reaching buyers is the harder part. And how to do that depends on the book. Non-fiction marketing is very different to fiction. The basic thing is to get your book where the readers who will want it can see it. For a permaculture book, that would be listing the book here, putting it in your signature, commenting actively on related posts, guest blogging or other related sites, sharing what you know and just "by the way, I also have a book on this." I also know authors who do good business selling their books at farmers' markets, even fiction authors. Having a book table at the back of the room at workshops. There are so many different ways to promote books, but it's not one-size-fits-all.
I feel the main way to earn an income is to follow your passion rather than doing something just to make money. A bit of a cliche, but your love of what you do will attract buyers for your product. And just like any good homesteader, it's wise to diversify. Aim to have multiple income streams.