runemeqoso

I collect rare books, and my main source is thrift stores and estate sales. Books are super weird when it comes to value. There are really two types of book hounds. The ones that go into library sales/thrift stores/garage sales with scanning apps and quickly scan as many books as possible, and anything that matches their predetermined "worth it" value gets put in their cart. And then there are the hounds that look for OLD books, such as myself. I have tried mass sales book hounding with a scanner and did not enjoy it. What I do enjoy is going into a thrift store and glancing over the shelves, looking for OLD books, like you can visibly tell are at least from the 40s or older. I've found some very rare and valuable books by doing this. Mainly because if a book has no barcode or isbn, a mass sales book hound won't even bother looking up their value. Why I enjoy it so much is because while looking up the value of these old books, I also get to spend time learning about their subject and author.
I still keep an eye out for newer books that I know have value. I know a lot about the sci-fi/fantasy book scene, and when I see a well known author, I immediately check to see if it's a 1st edition. You'd be surprised on the value of some 1st edition sci-fi/fantasy books, especially when they have a TV show that's become popular. I've made bank on GOT by purchasing a bunch of 1st edition ASOIF when I saw the trailer for season 1. I let the books sit on my shelves for 3 seasons, and sold them at the end of season 3 for a big profit. I don't do "investment" books anymore because it's always a gamble, and I mostly collect for a side hobby, but for those interested in collecting books, it's can be profitable. Also signed copies can really increase the value of a book, but not always. Dead authors signatured works are of course going to fetch more than an author still doing book tours. So keep an eye out for popular books, and skim through the first few pages for a signature.
I also have my own personal "not for sale" collections going on. Usually they aren't the most valuable collections, but they mean something to me. Things like an almost complete Terry Brooks 1st Edition collection, a Piers Anthony complete Xanth collection (edition doesn't matter, I just want to hound all the books). etc.