I wouldn't worry about selling someone a defective, dangerous board. It's only the footpad that is defective, and that is a $150 fix. They have been known to malfunction due to water damage, so if you have ridden exclusively on the beach, that is the likely cause. It doesn't have to have been submerged, Onewheels are often not very water resistant at all. It doesn't help that Future Motion has often posted videos of people riding in conditions that void the warranty. While it is fun to ride on the beach, salt water and electronics do not mix.
The heel lift is also key. Don't step off until the motor has disengaged.