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.
Elaborate, properly constructed spreadsheets greatly simplify a wide range of activities and processes within companies, enabling business owners to economize valuable time and effort, increase the efficiency of their business (Analysis), keep track of different tasks, manage their budget, optimize the planning of future events (Forecasting), organize and access valuable information, and optimize the completion of specific tasks. And thus, thanks to their many benefits, spreadsheet programs have come to be considered indispensable tools for businesses in today's competitive climate.
You'll need to find a way to make your spreadsheet accessible. You can build a website, which will require paying for a domain, hosting, site security, dealing with an internet transaction service, graphic design, content for the website, marketing your website, Search Engine Optimization, and all the other madness that comes with managing a site.
All of these things pull you away from the thing you really want to do...which is to build and sell great spreadsheets. The best thing to do is use Excelville.com since they have web developers and internet marketing specialists to help get exposure for your spreadsheet.