Questions about Fannie Mae REO

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Hi, I have been looking at a Fannie Mae REO and was doing a bit of research on it. I searched the county records and found the amount that FNMA bought the property for at Auction. It was around $50,000. Now the property is being listed on the FNMA website for around $20,000. Why would FNMA purchase the property at auction and then list it for less? Is FNMA required to list the price at FMV initially? How do they determine FMV?

Thanks for any help