Selling spreadsheets helps you to build a catalog of spreadsheets that you can take with you and use in various areas of your life. Spreadsheets built for sale are usually built for flexibility and would be customizable to your life, or job, or new life, or new job.
The common feature among the spreadsheets templates is that they save time and effort and also they make the work and other calculations easier. Planning out things and fast access to data and other information are made very easy when using spreadsheet templates. But most of these templates are unique according to the purpose. The templates can be saved and used as many times as required.
You'll need to include more than a basic level of development and calculation in your spreadsheet. There are lots of free spreadsheets on the web. Most of which are very simple and easy to build. People aren't going to buy a spreadsheet that they can get for free. Take yours to the next level by adding all the value you can imagine. Then research on how you can offer a little bit more on the development and functionality side.
If you have Microsoft Excel, you can create budgeting spreadsheets to help you manage your money. Instead of buying expensive software that will only be used for your budget, spreadsheets can be used for a variety of tasks. If you don't have Excel, you can find free spreadsheet applications online in which to manage your budget.