The cool thing about this particular Etsy shop is that it's run by booksellers who run a physical bookstore that they opened in Alabama in 2012. So really, this is the closest you can get to browsing a mom & pop bookshop from your computer. The shop owners source books from estate sales, thrift shops, flea markets, library sales, and more. They have a huge inventory, and what's listed on Etsy only represents a small part of what they have available; in fact, they encourage customers to contact them if you have a particular book you're looking for.
When you browse the Ol' Curiosities & Book Shoppe's Etsy storefront, you can browse books by categories such as children's books, law books, photography books, and books by Pearl S. Buck. But the bulk of their books are difficult to categorize as they are unique and vintage items. Most of the shop's listings are very brief: it's clear to me that the customer base is people who are looking for specific titles/authors, or people who will find familiar titles/authors while browsing the shop and feel inspired to buy. Who wouldn't be inspired to buy a book you loved in the past, or maybe a book that reminds you of a beloved person or place from your past?
One last thing: the prices are quite reasonable, many around the $5 mark. You can explore an old book for what it would cost to buy a couple of lattes. I got the distinct feeling that Ol' Curiosities & Book Shoppe is both a carefully run business and a labor of love from people who know and deeply respect the book culture of yesterday and today.