Depending on the type of scooter in question, buy used three-wheelers from Cycle Trader or from American Wheelchair's website. These are two very different kinds of scooters, but each of them has three wheels for added stability.
Cycle Trader specializes as a marketplace for the sale of motorcycles and transportation scooters. As of 2015, it hosts numerous ads and user listings for three-wheeled scooters, called "trikes" in biker parlance. Some are electric, while others are gasoline engine.
For used three-wheel mobility devices meant for seniors and the disabled, the American Wheelchair website hosts listings. Like Cycle Trader, it offers a mix of new and used scooters. Most of the devices are electric.