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.
You'll need to develop good marketing material. Things like demo videos, which you can find plenty examples on YouTube, screenshots, and keyword rich descriptive text help people find your spreadsheet and communicate the value that sets yours a level above the rest. Excelville.com actually allows you to upload your spreadsheet with all of this marketing material in an easily searchable and user friendly website.
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.
You could make money doing something you love (or like [or maybe not so much but just really good at]). It's great to see a spreadsheet come together, and usually a lot better than what you originally envisioned. Especially when the dang thing works.
types of spreadsheet
spreadsheet templates for business
uses of spreadsheet