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.
Today, there are a large number of options to choose from. The downside to upsizing your spreadsheet to a database system is that databases require fairly in-depth knowledge, especially with regard to extracting data in queries, form construction and reporting.
For pure data analysis, the spreadsheet is, indeed, king. To a point...Despite all of these extremely powerful features, the spreadsheet has limitations, which tend to manifest when you least expect it. Often, Murphy's Law applies and the spreadsheet falls over when you are right in the middle of a major undertaking, like month-end or meeting a tight deadline.
Using Excelville.com, there are no costs associated with selling spreadsheets and you don't need to build and manage a separate site, which works out pretty well. Whatever path you take, I wish you the best of luck. Leave a comment and let me know how things are going for you.