Prices for everything are affected by supply and demand. This is true whether you are talking about gasoline, art or residential real estate. If lots of people want something that is in limited supply, the price goes up.
The National Association of Realtors tells us that the supply of homes for sale increases dramatically every spring. If you put your house on the market now instead of waiting for spring (and increased competition), you could be well ahead of the game.
Buyers that are canvasing homes for sale in the winter are highly motivated buyers. They want to buy now and, because of the limited inventory, a seller has a stronger position from which to negotiate. Think about it. It makes a lot of sense.