Don’t write in a vacuum
There are plenty of gifted writers, producing some great stuff. But publishing is a commercial business and often authors forget this. Writing can simply be a fun, creative pastime, but if you’re serious about selling your books, you need to think more like a publisher.
What do we mean by writing in a vacuum? Some writers will set about writing a book without considering whether the topic has already been addressed by someone else. Or worse, someone else has tackled it much better than you. Worse still, a well-known, bestselling author (who will be very difficult to compete with) has tackled it better than you.
Writing in a vacuum is writing without market awareness. Do your homework. Make sure you know who has written what on your chosen topic.