purchasing a foreclosure

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Hello,
I am not an investor, I just want to purchase a home.
we came across an ad for purchasing pre-foreclosures
placed by an investor(?). I see that it seems to be a
legit profession. This man was talking about assumable loans, and my question is if we don't pay a downpayment and we assume an already existing loan,
how does the investor make any money? I guess it is because of my inexperience I am afraid of being scammed. Can someone please give some advice on how to avoid being taken to the cleaners?