Market works only when there is sufficient information. If I am selling you a car and you do not trust me, then you can only offer a lower price for it as you have to presume that inside is rotten. Then, an honest seller can not sell a good car on the open market, and the only sellers will be those with cars rotten inside. Thus, the “used car salesman”.
As mentioned above, you need to show it to a disinterested car mechanic – pay him well for the estimate and make clear that you are not going to bring it to him for repairs.
The same rule works in other fields where buyers can not fully evaluate quality of the product – universities, schools, doctors ..