Chessman - -
Here's some advice I hope you find useful. As a business owner I can tell you, if you are relying on others to tell you something is profitable, or that there is a market niche for your idea, or what the demand is for your idea, or what the "best way" is to market add-ins, as you have posted, then I'd recommend you not bother with the endeavor because your thought process is too reactionary.
You need to change to a more proactive "I am just going to do it" drive to succeed, if indeed your idea is a good one. Now, common sense and wisdom are important too, because if your idea is to market the next Edsell, then it's a doomed project.
I understand you are not talking about Edsells, you are talking about Excel add-ins. Instead of wondering out loud to a broad audience, do your own research and target your questions to the people who develop and sell their add-ins if their revenues are what they expected, what they would have done differently, and what they did that worked. There are many web sites with many marketed add-ins. Some of them have telephone numbers that you can call to speak with the Excel developer who is selling the add-ins. Many business owners will respond graciously with useful tips if they don't think your goal is to undermine their operation. We also admire in other people the traits we like to think we have, which is, we had an idea for a business, we believed in it, we left the steady job and income (such as that is these days) in favor of taking a chance on what we think will be a more satisfying lifestyle. You may not make as much income selling your add-ins but when you wake up in the morning, at least you will know what mood your boss is in. So in that respect, you should assess the trade-off between income and happiness, especially during the start-up phase when there is not much income, until you keep pressing and achieving market share. I work 16 hours a day but I love what I do, I enjoy dealing with all my clients, and my stress level is virtually zero (except at tax time).
Here's the main point. The world is full of products that are now mainstream but were not well received when they were first suggested. Federal Express; Post it notes; compact disks...the list goes on and on. Here is link worth reading:
http://www.fastcompany.com/online/33/one.html
You have an advantage of other people's experience with the same idea for advancing Excel usage. So, target your inquiries to those people who do what you are considering, and still, trust your instincts to move ahead without fear of failing, until common sense tells you that maybe, your idea was not as good as you thought. Even if you fail, something will be learned from the process and you can succeed next time, because successful business enterprises are usually not made to be successful on the first try; it is truly a learning experience.
Good luck.