If your next vehicle will be a minivan, you may or may not want to get something that is used. New minivans are often very expensive, so a better option would be to go for used minivans.
There are many websites from where you can buy this vehicle. For example, if you want to get a used minivan for sale check out https://www.carsoup.com/for-sale/Used/Mini-Van/. But you need to check them thoroughly before buying.
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When examining them, there are some things you should insist upon, and some extras that you may want to have when you make your final choice. Most of the people who choose these cars have children, so every old car is not necessarily what will work best for you and your family.
Just like any other car, always check the used minivan for rust. If you live where winter is a pain, road salt can do a lot of damage to the bottom of any vehicle. Look under the minivan perhaps before you decide to do further checks.
If you see rust, you might want to move. You should also check used minivans for doors that will not work slide. This is often problematic in a used minivan, so it is always something that you should check before you get too far. If they do not work, you want to look for something else.
If everything looks fine so far, you can then check the minivan used as you would other types of vehicles. You should see the mechanic on the engine and all the relevant mechanical parts of the minivan.