Which ones sell the best? Technical? Financial? Textbooks?
There is a thrift store near me that he a book stock pile that rivals that of a small library. Maybe I can cash in a little and make some tax free income
Oof. That is a complicated question, because it all varies depending on what sells on Amazon. If this is something you want to do, I would get a subscription to a listing service (I use ASellerTool when I'm doing this; my enterprise is currently on hold) and update the listing data right before you go to the Goodwill.
I have had good luck with obscure, graduate-level textbooks and works of non-fiction; non-fiction trade paperbacks; weird or old art books; books on practical subjects, like hunting or internal combustion engines; and "creepy Jesus books," which are mostly works that are Bible-adjacent and still bear stickers from Family Christian Stores. I would avoid almost all hardback novels, romance novels, anything financial, and pretty much all mass-market paperbacks.
I spent about $200 for start-up costs, which were for a Dell Axim x51 pocket PC and a barcode scanner that plugs into the top of it. Then I mapped one of the buttons to trigger the scanner and set my buying criteria within the program. ASellerTool has a lot of good information on how to do this for yourself; if you're at all interested, I'd check out their website and see if it's an option you want to pursue.
You'll also need a way to ship the books to their intended recipients; I use mostly #1 and #5 poly bubble mailers from ValueMailers.com. Amazon lets you print postage directly from their system, and if you do that, you don't even need to stand in line at the post office.
I treat this side hustle mostly as a way to enhance my own library, since I usually find at least a few books on each trip that I want to read, and selling the "good" books that I find more than makes up for the $3 or $4 I'd spend on books that I want to read. Plus, once I'm done with my own selections, I can cart the ones I don't want to keep up to Half-Price Books and get even more cash back for them! And then, of course, comes the thrill of finding something that sells for a LOT of money. I found a calculus book once that I bought for $2 and sold for $60 within 24 hours. A book on internal combustion was acquired for $2 and went for $120; I bought one on quantum physics for $1 and sold it for $80; and the best one was a book about Elizabeth Taylor's jewelry that I had picked up on clearance at Barnes and Noble, but went for $225 to someone in Australia.
I call it "book-hunting" and it's a blast. I always go for the books first when I go to a thrift store, I have a weakness for them. The only thing is, now I have a problem with paying full price for a new book. I guess that's not entirely bad, though!