If you love the art of the deal, you might be accustomed to buying consumer goods secondhand. A car, a gently used coat, a vintage pair of boots or an antique timepiece doesn’t need to be new to be safe, useful or even cool. But there are a few things you should never buy used. A mattress is one them, and here’s why:1. You don’t know how a used mattress has been cared for.Have you ever driven down the highway, in the rain, and passed a truck with a mattress in the back? How do you know that’s not the mattress you’re buying from a resale shop? The most problematic aspect of a used mattress is the possibility that it has been damaged, or worse, infested with something like bed bugs. Even critters that pose a less disturbing problem (like dust mites) are difficult to get rid of. A mattress can’t be washed and sanitized the way a pair of jeans from a vintage shop can be.2. You can’t evaluate the quality of a used mattress. Even if a used mattress looks clean and has only been slept on for a couple of years, it’s hard to evaluate the current state of the mattress. Lower quality foam, springs and other materials can deteriorate over time. Body oils, body fluids and spills can significantly accelerate that deterioration, but may not be immediately noticeable upon inspection.Unlike buying a pre-owned car, where you rely on a mechanic to evaluate the engine and purchase an extended warranty, a used mattress is difficult to assess.Most new mattresses also come with a warranty. So, buying a used mattress puts you in a double bind: You’re more likely to encounter a problem or defect in a used mattress, and you won’t have a warranty to cover repair or replacement.3. A used mattress has a short remaining lifespan. A mattress is typically good for seven to ten years when well cared for. If you buy a used mattress from someone who slept on it for three years, even if there are no mattress defects and it meets your criteria for comfort, you’ll still need to buy a new mattress in a few years. Why not start fresh with something you really want?4. A secondhand mattress won’t be customized to you.When you buy a new mattress, you get to customize the mattress support system and the mattress comfort attributes. To help you with this process, the Serta mattress selector takes your preferences and lifestyle into account and identifies your best mattress options. It’s highly unlikely that a used mattress would meet all of your comfort criteria.5. New mattresses are made of better materials.The same way software and cellphone technology evolve each year, so does mattress technology. Mattress materials, like memory foam, continue to advance and support a more dynamic range of comfort preferences. For example, memory foam was considered a warm material for many years. But Serta’s exclusive gel memory foam is an advanced material infused with millions of gel beads that respond to your body, providing better support and cooling comfort.