Rare Retro Style 4 Bit Computer (powered by 2 AA batteries) - 4 bit CPU, program in binary and assembly. Learn how data moves between registers and how operands and opcodes work. Be the closest to the metal that any hacker can get outside of designing his own computer from the CPU upwards. There is an entire website dedicated to this product with easy to follow instructions, code examples and various tools (search on Google: 2022 Hackaday Supercon 6 Badge Guide). There is also a video that is full of amazing programs people have created (search on YouTube: Hackaday Supercon 2022: Badge Hacking Awards Ceremony). I would list the links but eBay does not permit embedding links to any outside websites.