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.
At present, some of the most requested types of spreadsheet packages in the market are: 'Spreadsheets for Small Businesses' and a subset of that: 'Spreadsheets for Actors'.
One user at a time : In the instance where there are a number of people required to work on a spreadsheet, they can only access it when no-one else is working on it.
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.