...is that you would want to offer resell rights if:
1. Your ebook is standing on its last leg in the sale department. Offering resell rights to select people for a high fee is essentially cashing out.
2. You want to be a loss leader. Meaning the book is intended to be promotion to sell another back-end product. Tom Hua of the "Free To Sell" packages creates packages of resell rights books to promote his back-end products or promote lucrative affilliate programs.