Resale rights have become popular with the popularity of the Internet. A very similar idea called reprint rights existed before the Internet. If you owned reprint rights to an article, a piece of music, or a work of art, you could make copies of the work and sell them. When the Internet became popular, this idea was translated to the electronic medium, and since printing was no longer required, the term resale rights became more common.
Resale rights allow the owner to sell or resell a work to others. This is very common with e-books. You can buy an e-book on its own, or you can buy an e-book and resale rights.
If you only purchase the e-book, you can only use it yourself. You cannot sell or give the book to others. If however, you also purchase the resale rights, then you can also resell the e-book.
Very frequently, resale rights are included in the purchase of an e-book. This may give the purchaser more value for the money spent. The purpose of many of these e-books is to get as wide a readership as possible.
Many e-books contain sales pitches for other products. It is the author's intention in many cases to make money from the sales pitches embedded in the e-book rather than from the e-book itself. Sales from the e-book are a bonus.
Resale rights are also important to freelance writers. When an article is written and sold, it is actually various rights to the article that are changing hands. The purchaser of the article may insist on and pay for exclusive resale and reprint rights. In this case, the author may not sell the article to another publisher.
The purchaser may also purchase non-exclusive reprint or resale rights. In this case, both the author and the purchaser are free to sell the article to any other publisher. The author may also sell one-time print rights, in which the publisher may use the article once and cannot reprint or resell it without further permission from the author. A publisher must pay more for exclusive reprint and resale rights than for non-exclusive or one-time reprint or resale rights.