Sorry Tom, I hate mysteries but every leak has a source. Without insulting anyone you've got to clean everything around the area perfectly and be suspicious of any wires or hoses passing near there, liquids can travel unseen long distances on undersides of hoses, wires and metal or plastic surfaces. Be sure exactly what the leak is, a little oil mixed with anything can throw you off. Run it only ten seconds then look and WAIT. Look again in the morning. Does it leak constantly or only after it runs? Take pictures from angles you cant get your face to see. This forum is full of really clever guys who are just like you and none of us enjoy defeat. Did you change the head gasket? Have a friend look at it, sometimes we miss the obvious. We have all been there but I bet if you find the source one of the clever people here will help you stop the leak and then if you want to sell it youre not apologizing.