Letgo — sennheiser pxc 550 headphones

This concerns the purchase of a used headphones set from and individual seller in letgo. the seller said the headphones were in perfect condition and had not been used about 3 times. I negotiated down the price based on the fact that he was selling with absolutely none of the accessories. When we complete the transaction I looked over the headphones and they seemed okay with very minor cosmetic damages. Then when I took them home I noticed they had a strange smell like someone had sweat in them. I also took off the earcups and found the mesh protecting the driver was covered in skin flakes, dust and earwax. They were clearly obviously used many times and not clean or hygienic. I'm not okay with the fact that I was lied to, and the seller is being very evasive about finding a solution. I would like a full refund. It has been 2 days since the purchase and the seller has admitted wrong doing. What can I do?