Gather all the information you can on the item you want to sell. For cars, the make, model and mileage is important, as well as condition. For houses, record things such as the number of bedrooms, baths, lot size and square footage so you can select comparable houses.
Research similar items online. For home resale value, check out recent home sales in your area. For car resale value, check out sales of vehicles of similar make, model and mileage. The more closely the items you are comparing yours with, the more accurate your calculations.
Write down a minimum of five sale prices for items that are comparable to yours on a sheet of paper. The more data you are able to gather the more accurate your calculations.
Add all the sales prices together and divide by the total number of items to get the average sales price. This will give you the resale value of your item.