This is an Aeris Swopper stool (https://www.aerismotion.com/products/aeris-swopper-recycled-polyester?variant=51337847996709). They cost between $800 and $900 brand new, but I've had this one for a few years. The bottom paint was damaged in a move, so I've lowered the price to $150.
The stool works perfectly. The height is adjustable, and the seat spins. The central beam moves with your body, helping to keep your spine aligned, and it helped me recover after a back injury. Pickup only