Buying a New ATV
Buying new is always nice. You get that new quad smell, you know what you're getting, (including a warranty!) and there's a certain satisfaction that goes with a brand new ATV. They're more expensive, but there's piece of mind that is often worth it's weight in gold.
If you've decided to buy new you should shop around. Check with all the dealers in your area and don't be afraid to play them against each other. Don't buy something on your first time out, it will let the dealer know you're not going to be pushed into something you really don't want.
Don't Let the Dealer Make Your Decisions
One thing dealers try to do is sell you what they want. That usually means getting more than you wanted in the first place. When you buy a new ATV you should get what YOU want. If you're not sure what you want, it is best to visit as many dealers as you can and look at as many different types of ATVs as you can.
Once you've made your choice on what to buy, start thinking about how much you're willing to pay. Most dealers will deal with you to get the sale, so try to get them to lower the price or at least throw in some extra goodies with the sale, like accessories or some ATV safety equipment.
Buying a Used ATV From a Private Party
Buying used is great if you're on a low budget, if you're a first time buyer, or if you know who owned it previously and you're sure it was well taken care of. If you know someone that knows motors and ATVs, they might be able to help you find a used ATV that's in good condition.
One thing to watch out for is damage from a crash. Look for a bent/cracked frame, rust, loose parts, and dings/dents in the rims or under-carriage. Also check the steering stop for marks that would indicate it was slammed on the front wheels either form a jump or from crashing into something. A compression check is also important.
Buy a Used ATV From a Dealer or Mechanic
Sometimes you can't get the quad you want new, and don't want to take a chance on buying used. Consider buying from a reputable mechanic in your town or a dealer. They often get ATVs in trade and re-sell them after testing and fixing many possible problems that can come with a used quad.
Buying a used ATV from a dealer or mechanic is a good way to get a better price on an ATV that is usually going to be in better condition than if you buy from a private party. They know what to look for and will take care of any major problems before selling the ATV.