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.
Protection of formulae : How often have you forgotten to protect the cell contains the formula that took you hours to test and perfect. It takes one instance of honest human error, and that formula is gone.
Spreadsheet applications are useful for home and small businesses. Some people keep a home inventory list on a spreadsheet and others use spreadsheets for a variety of applications. Creating your budget and maintaining it is easy when you use budgeting spreadsheets. These are also easy to print out if you desire.
Size limitation : When a spreadsheet is first constructed and tested, the size tends to be small. However, as the spreadsheet grows with more and more data, it start slowing down, until it reaches that point where your feet go to sleep while you waiting for the re-calculation to finish.