When buyers enter your home the first impression they should have is of light. Sun shining through windows. Lamps on in dark corners or basements. They should never walk into a dark room.
Take a look at your window coverings. Do they obscure the light coming into the room even when they are open? Remove any extra curtains or blinds. You should only keep the most minimal, lightweight window coverings needed for privacy. Be sure to open all draperies and blinds before you leave the house.
This one was recommended by my realtor and it really made a difference: remove the window screens. This may not be necessary with the new, lighter weight screens, but classic weight screens need to go. Try it in one of your rooms; you will be amazed at how much light a window screen prevents from entering the room.
Wall color must be a neutral, light color. I'm not recommending that you paint all your walls white, though white might be appropriate for a very small, dark house. We don't want the buyers to need sunglasses! Get rid of the dark paints and very taste specific paint colors. Remember that the buyers will look at your red walls and think about how much it will cost them in time and labor to repaint.
Trim shrubs and trees that are preventing light from entering your home. They may keep your air conditioning bill down, but that's not a consideration right now. Your windows should never be obscured by plantings.
Clean your windows. All that light will highlight even the smallest amount of dust on the glass.
Remember, winter is coming and there will be very little natural light available on some days. Do you have a dark corner that the sun never reaches? Put a lamp on a table in any dark areas and turn them on before the buyers come over.
This is an easy fix. And easy on the budget. Isn't it worth putting hundreds of dollars into increasing the lightness of your house in order to make thousands more in profit?