You don't 'unknowingly' buy a house. Sounds more to me like they
thought they had a steal that they could do some minor renovations and sell for
a large profit, and they were to cheap to do their due diligence. They should
have done this test PRIOR to buying the house. They also could have gone around
and talked to neighbors to gather more info about the house and its history.
They also could have contacted local agencies to inquire as to anything out of
the ordinary about the house or prior residents. They also could have...I don't really feel bad for them.