Bicycle helmets
Bicycle helmets are designed to protect us in case we fall but they do not last forever. They are made of Styrofoam which will eventually get damaged and even break during a major collision. This means buying a secondhand helmet might not be the smartest idea.
You don’t know what the previous owner did to it while in their possession, which means it can be damaged without any visible signs. Obviously, even a used helmet is better than no helmet at all, but the safest and cost-effective way is to buy your bicycle helmet from a trustworthy vendor.