The absolute biggest bang for the buck is to clean the house and then clean some more. This goes along with getting rid of clutter. Once that is done make sure the grass is cut, shrubs are trimmed and the general curb appeal is attractive. Basically there is no investment of money, only time if you do it yourself. No question it's the biggest bang for the buck.
Once everything is cleaned up you can look at painting and removing wallpaper if you have it. Muted colors with contrasting white trim are popular and can give a house "pop" when you enter. Again, great bang for the buck.
If you have hardwood floors that need refinishing that would be the next step. If you have carpet that is not pristine and hardwood floors underneath, then I would take out the carpet and let the hardwood shine.
A number of agents mentioned new kitchen, bathrooms and roof. That might help sell the home faster, but it's rare you will get any profit for the money spent and more likely you will not get back your investment and only a percentage of it.
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