Banks are not charities. If they foreclose on a home and end up owning it, they are under no obligation to sell it at a discount. In fact, they are under an obligation (to stockholders and regulators) to sell it for the highest price possible. Just because the loan balance due was $30,000 (sounds like a second or home equity line of credit, but I'll pretend that is the total of liens for now) does not mean the bank will sell the home for $30,000. If the loan balance due was indeed only $30,000, and the home was worth roughly $500,000, it would most certainly get bid up at the Trustee's sale, and practically speaking the bank would never have to own it.
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