If you buy from a private seller, for example a person selling a sofa on a free ad website or in the local paper, you do not have as much consumer protection.
The goods you buy only need to be as described. So if the seller says something is in mint condition, then it should be free of any defects.
Beware of rogue traders pretending to be private sellers. They may do this to avoid complying with consumer law. If you think someone is doing this, you should report them to Trading Standards Service via Consumerline on 0300 123 6262