For many types of durable consumer goods, "shop secondhand" is a valuable money-saving tip. Books, cars, dishware, décor, and most clothes can often be bought for pennies on the dollar compared with their normal retail prices. But sometimes buying used can be a waste of money or even be dangerous. The following items should always be bought brand-new, either for safety or hygienic reasons.
Modern safety helmets are essentially "single-use" items, designed to absorb the impact of one crash. After that one crash, the helmet is useless. But most, if not all, of the damage is to the helmet's internal structure, invisible to the naked eye. It's impossible to know the history of a used helmet, so buy a new one from a reputable retailer.
These items were worn in too-close proximity to their previous owners' feet and nether regions. New garments can already be bought so inexpensively that the savings would be negligible.
How clean was the previous owner's hair? There's no way of knowing. Machine-washable knit hats might be safe to buy used, but fedoras, cloches, and other wool or felt "shaped" hats generally are not. Such hats must be cleaned professionally, and the cost would likely negate any savings compared with simply buying the hat new.
Many forms of software require registration with the maker and can be registered only once (or to a single person). Many software packages, such as the Rosetta Stone language programs, expressly forbid resale in their licensing requirements.