8 Things You Shouldn't Buy Used

A helmet that has been dropped or involved in an accident won't protect you the way it should—even if it looks fine. Get professionally fitted for a new helmet (free at most bike shops), and you'll have a helmet that is both safe and comfortable to wear.

Most car seats expire six years from the date of manufacture, and while the expiration date is easy enough to check (it'll usually be printed on one of the seat's labels), it's still a good idea to steer clear of used car seats. They may have been dropped, involved in a wreck or otherwise damaged—there's just no way to tell.

Still pained by the thought of springing for a new car seat? Just remember that a new seat will include all of the latest safety features, so this is one time when buying new means getting more.

Mattresses have an average lifespan of just five to seven years. You might just end up with one that no longer provides proper support and comfort. They also come pre-loaded with dust mites, dander and other things that you probably don't want to think about—and may even be allergic too. 

To save money on a mattress, buy new; then, flip and rotate your mattress as often as the manufacturer recommends. It's an easy way to get a couple of extra years out of your purchase.

Some things just aren't meant to be passed on, and unmentionables definitely fall into that category. If you find yourself contemplating the purchase of a used swimsuit, consider this: fungus (as in the kind that causes yeast infections) can survive in clothing.

Buy a pair of shoes that someone else has broken in, and you could end up with foot problems that cost more to treat than a new pair of shoes. So not worth it!

When to break the rule: If you come across brand new or worn-once shoes when you're shopping at yard sales, thrift stores or flea markets, and they're a top-notch brand, go ahead and buy them.

Buying used makeup could turn into a buy-one-get-one-free deal, as in: buy a tube of mascara, get a free eye infection. Since you really don't know what's lurking in that makeup container (or even how old it is), it's best to start fresh.

Bad things happen to good laptops. They get dropped, spilled on, infected with viruses and left in hot cars. So, that "like new" laptop that you're looking at may keep chugging on for years, or it may crash tomorrow. Nobody knows.

When to break the rule: If you find a manufacturer refurbished laptop that meets your specs, buy without fear. Even if something goes wrong, you'll be covered by a warranty.

Tires are expensive, so people usually wear them out before buying another set. If you find a killer deal on used tires, there may be a reason someone's ditching them early. They could be out of round, noisy or have poor traction (none of which are problems you want).

Another issue: Tires dry out over time, so if you don't know how old the tires are (and how long they've been off of a vehicle), that's another reason to be wary. Ford recommends replacing your tires every six years—even if the tread is good.