If a seller owes $50,000 on his mortgage can't I just pay him that $50,000 to pay if off so I own his house?

Reason I'm asking this question is because. I was thinking in my area land lots are selling for about $60,000-$80,000. Why don't I just find a seller who can't pay off his mortgage and pay off his $20-$50,000 mortgage that he still owes. Wouldn't that mean that I just purchased a landlot for $50,000 or less? What I want to really do is get a house for that cheap and just do a complete teardown and build a brand new three family condo complex on it. I'm an amateur at this so just wonder why this can or can't be done. Most people own really ugly houses anyway which they still owe the bank alot of money which they can't pay for. At least in my area which has high property tax. Most people I think just want out of their mortgage. Then I could make a deal with the seller to pay them the rest of what they have already paid into the property(Equity). After I have sold all 3 condo units. Minus 20-30%? Basically I'm asking the seller to be my partner. Condos in my area are selling for a ridiculous $150,000-to even over $200,000 each and most of those aren't even in those huge condo complexes with elevators and doormen or private parking and security or laundry rooms. So a lot with a single family house worth $200,000-$300,000 could easily net double that. Plus the 20%-30% I get from the seller. I'm realizing now that this is the basic proforeclosure deal right? But my scenario is involving the seller in the new 3 unit condo construction.

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