The sale is made after the development, not before. To build a successful application you need to know the field you are designing for or work with someone else who does. If you don't, it will be difficult to act as a customer. You should consider tackling something with which you are more familiar. A Vertical Market product has a design phase, just like a custom application and involves many of the same problems.
Feature creep is as much an issue as in custom application and a good overall design is paramount.
Vertical Market products have different technical issues that can be more adversely affected by numerous design changes than a custom development project. The Technical issues go hand in hand with Design issues. Numerous Design changes cause numerous code rewrites and form changes. When designing Vertical Market products, everything needs to be more flexible. Some users like integrated Help files, others never touch them.