Can you buy a home that has a mortgage on it?

Lets say there is a home valued at 150k and has a 50k mortgage left. I know that if I buy it from the sellers for 150k then 50k will go to the bank and 100k to the sellers.

But what if the home was refinanced at some point and now there is 170k on the mortgage? Since the home is only worth 150k then can you even buy it? The sellers would be 20k in the hole, right? Would it even make sense for the sellers to try and sell it before they paid off more of the mortgage?

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