The baby bouncer market is a competitive one, and baby products can be produced quickly. This means that used baby bouncers may not have been tested or approved for safety purposes. There's also no guarantee about what types of materials were used to make the baby bouncer itself since there could be various harmful chemicals in older models.
In addition, baby products are often recalled by manufacturers if they're found to violate standards. It's best to only buy from trustworthy companies that will provide good quality baby products rather than risk your child's well-being because you bought something second-hand!