Remember... not all Ebooks have resale rights. In fact, most don't.
Even if you bought an Ebook as part of a resell rights package, and the owner tells you that the product is resalable, triple check to see if the product is really allowed for resale.

Many people blindly take other people's words and confuse and upset people unintentionally because they sold something that wasn't supposed to be.

In general, you can find the legal notice on the first page of an Ebook.
Resale rights come in different ways...

? You can buy a product with resell rights, and then resell the product and not allow others to resell it.

? You can buy and resell a product and allow your customers to also resell it from their own site.

? You can buy a product, and allow anyone who buys it from you to sell it, including selling the resell rights.

This is best known as "Master Resell Rights", or also "Generic Resale Rights".
Beside resell rights there's also something called "Customization Rights".
Some products allow you to customize the affiliate links inside the product with your own affiliate links to earn backend profits.

Mostly this requires a separate fee. If you buy "customization rights", you can insert your own profit generating affiliate links in the product, and anyone who buys the product from you and resells it, will help to spread your affiliate links.

You won't find this option often because most authors want to earn backend profits for themselves, and not give them away to you.
The major reason that people sell their products with master resell rights, is to spread their name so they can make backend profits on it.

A backend profit is a profit that you make after selling to an existing customer. Example: you buy an Ebook for a low price and then the seller wants to sell you an additional product of more value later.

Front end profit is the profit you make from your first sale to a customer. In other words... if you make a second sale to the same customer, that's a backend profit. That is where the real profit is made.

If a person buys from you once, they will buy from you over and over again - at least, as long as you deliver the same quality.