Be smart about the purchase that you make. If you are going to use your ATV for work only, you don't need it to be flashy or fancy. Less frills means you will be able to find an ATV for cheap.
There is always a risk factor when buying anything used. You don't want to buy a part only to have it not work properly. It's important to feel comfortable knowing that there is a clear return/exchange policy and some kind of warranty. Have a firm understanding of the warrantee before making a purchase. You don't want to get scammed because you didn't read the fine print. If they don't even offer a warranty, find a different place that will.
Especially for older cars or antique vehicles being restored, used car parts may very well be the only answer to your parts needs. You may not find any auto supply outlet with access to car parts for a '49 Studebaker pickup or a '32 Ford, but you may very well find auto salvage near my location with cars that are being sold for parts.
Decide what you want to spend for the part before you go. This can keep you from overspending on your budget. Sometimes these junkyards have staff who will pull the part or get it off a shelf. If you know what you want to spend it will help you determine if you want it from this place at all. Also, find out what they will charge you for it if you find it and pull it yourself. You don't want to go through the hassle only to leave it behind because their prices are out of reach.
Determine Your Need: Every spare part is made for a specific make and model of a BMW, Mercedes, Toyota, etc. It may not be compatible with another make or model. Therefore, wherever you will be buying your auto parts, be specific with your order. Let the dealer know that you need x part for y vehicle of make a and model m. Failure to do this may cause you to regret later on. Once you have defined what you need, we move onto the net stage.
Never think that you are purchasing used parts and they are very cheap in price, so there is no need to look for a reliable store from where you will shop these parts. Purchasing fake parts will not provide you with any kind of benefit, but you will only waste you time and money. There is no shame in purchasing these parts. On actually using junkyard parts, you are surely going to recommend them to your friends to purchase these parts.
Know what you are looking for. The most important point is to know what you need. To many people set off looking for parts before they really know what they are looking for. Do your research and make sure you have a good idea of what the going rate is, how can you get cheap parts if you don't know how much they are!?