Seller listing th house on market

[Keri Mateir]

hi,
I live in a area where housing is holding pretty well.
I found a house where the seller has been served notice of trustee sale. The house is going to be auctioned. but the seller has listed this house on the market.
Can they do that? if yes then why not every seller list the house in my area where I know it will be sold for more than what an investor or I will offer them.
I have an example where there was a house short saled by an investor for 20-30% than market value. why did that seller did not listed?