In my area, very few sellers provide warranties of any kind and most resale properties are sold "as is". Only new builder sales offer warranties which they charge a premium for being new.
Not all REO properties sell for less and when they do sell for less, the reasons can vary per individual per market area and property type. How much "less" they sell for than non REO might also determine where market value is...remember , market value is NOT identified as the highest probable price, it is identified as the most probable price.
When REO and non REO owned properties compete, who is paying market value....the buyer who paid more , or the buyer who paid less? ( assuming properties are equivalent in features, condition etc)
A lot of buyer behavior is motivated by perceived risk and reward, not actual risk and reward. Of course an RE property presenting greater actual risk ( poor conditioning, liens, title cloud ), should sell for less. The remainder, much of it is about perception. Which is how property flippers make money.. buy a house with no real issues, maybe in dated cndiiton and REO owned...paint it, clean it, change out appliances and put it back on the market and make $ . Pretty sweet for property flippers. The opposite is builder new houses,buyers perceived by buyers as low risk because newer and comes with a warranty. Meanwhile many new houses lose 10 % to 20% of price if resold within a few years....not sweet for these buyers ;they took on actual risk without realizing it, over paying for what they though was the safety of new and a warranty..