A great example of a "one sale only" product is a book or ebook!
For example, I bought the book, "How To Sell Anything To Anybody" by Joe Girard. Chances are, I'll never buy it again - whether I loved it or hated it! That's because, once I've read it, I've "got" the information... I don't need a second copy of the book!
However... People who are happy with the book will recommend it to their friends. (As I did.)
To turn it into a "repeat purchase" product, however, you can write a series of books on related topics... Even Joe Girard did this (on a small scale), with 2 "sequels" to his first blockbuster book... Robert Kiyosaki has taken this model to the extreme. I count 25 books now in the "Rich Dad" series of books! (He should rename his series of books to - "Read till your eyes pop!")