Seller's mistakes. This is a pretty large territory to cover. There are just so many things that a seller can do, or not do, that can cost them money or the sale of their home. The one I see most often can be fixed with a good coat of paint.
I have time and again gone to homes and found the lovely paint scheme they have used. It was all the rage...In 1974! Or worse, the kids all got to pick the color of their choice for their own rooms. Once painted, they then stuck things to the walls or worse, put up glow in the dark stars all over their ceiling. When it came time to sell, the owners were either not interested in returning the homes colors to a nice neutral color, or else thought their colors would be appreciated by the next buyers. Sadly, this is usually not the case.
Painting the house a neutral color is probably the easiest and least expensive thing a home owner can do to get the biggest bang for their bucks. Take the time to freshen the paint and your home will sell for more and faster.