buyer of our used car now says something is wrong

my husband and i, from pennsylvania, just sold our 1994 ford taurus. the couple who bought it looked it over and started it up, but never took it for a test drive. they accepted our price and we transferred the title per their insistence that same night. they are now calling and told us they took it to their mechanic who says something is wrong with the engine and that no good is going to come out of our deal. we told them that the car had been sitting for over 3 months, never gave them any kind of guarantees--we did not know that anything was wrong with the engine--we had a motor mount replaced and was never told about anything else being wrong. the car had 110,000 miles on it when they bought it. if they want to go back on our deal, do we have to?
thanks