If you are considering not replacing your roof before putting it up for resale, you will likely see a significant drop in your house value. Most new homeowners want to buy a house they will not have to spend extensively on from the get-go since the house purchase itself will be a huge expense.
Therefore, if you want to drive up the house value, invest in a replacement. You are likely to get back the money and more due to the boost it gives to the price.