there is no one specific place which lists cars for sale for half the price.
you can start by searching on
www.google.com for "cars for sale" or "used cars" or check out some forums.
however, if you need to know if the price is right, or 'half', you have to do further research to find out the original price.
and beware, some owners may sell you their cars at half the price, but you will not know if there are any issues with the car, best to buy using popular/legitimate channels
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Used Cars For Sale|
Used Cars