I self-published 2 books under a pseudonym. They did not do well. How do I tell if they did not do well because I failed to market or because no one is interested? I'm trying to decide whether to invest a lot of money in marketing. I did get one nice review from a professional in the field that the fiction was written about. Also, the books were previously only available as trade paperbacks, which were over $20. I recently got e-book format available, and the price for that is only $2, so it might be a good time to launch some marketing. Or to go back and edit the things and repackage them, but, since I wrote them a long time ago, I'm not sure my heart is still in them enough to re-write them. I now have a third book that I am trying to market to agents. Should I use this same pseudonym, or should I concoct another?
If you've written two books with a particular pseudonym and they did not sell well, then there is not advantage to you to approach agents using that pen name. If anything, there is a disadvantage. So I would advise to choose a different name.
You ask if your two books didn't sell well due to your failure to market them or because no one is interested. It would be simplistic to answer that it was definitely due to one of these two reasons. There are dozens of reasons why a book might not sell well. First, of course, one has to look at the concept, the writing, the execution, etc. But assuming the writing is excellent, there are still many reasons a book might not sell, including the competition, the jacket, the synopsis, the categories, the pricing, the keywords, the timing, etc. etc. Even with excellent marketing, if a book is priced too high, or has a terrible jacket, or title, etc. it might still not sell well. There is no black and white answer, and it is not a science. But in my view, lack of marketing is usually the last reason a book does not sell well. Usually it's something else. I've seen many great books have paid advertising and not sell. And many books with no marketing take off on their own. $20 for a trade paperback is typically too high. And if your heart is not in your books enough to rewrite them, then that might be the most telling sign there. If you don't care enough about them to revise them, then it may be your readers don't care enough either. Writers must be completely devoted to their works, and willing to revise them countless times to get them to be as great as they can be.