"I'm only selling my ebook on Amazon..."
Self published authors or very small publishers sometimes choose to concentrate on supporting sales through a single retailer – usually Amazon – and will list Amazon as their supplier in their ONIX feed. Unless you really have no plans on supporting sales with any other company, this is atypical in ONIX and you should list yourself as the Supplier.
"What is a supplier?"
ONIX is a business to business metadata standard and the supplier is the company a retailer contacts (and pays) in order the sell the book.
It's easier to understand for print in that the supplier actually ships stock to retailers and processes retailer payments for the publisher. Tracking all that activity normally requires a focused business once you've published more than a few books. Typically that means the job is assigned by the publisher to an outside firm and that supplier supports sales "exclusively" to a market area. The publisher may still sell books to retailers direct but provides those orders to their distributor and the distributor handles the fulfillment of orders as well as accepting re-orders independently of the publisher. The distributor also handles the money supplied by retailers and distributes the appropriate share to the publisher. Distributors hold contracts directly with retailers and normally make all their books available to retailers for order (which doesn't mean a retailer has to order or stock them, the publisher still needs to represent their own books). Very large publishers have divisions within their structure that handle distribution the same way an outside firm would.
"...What about ebooks"
It's no different for digital products but the Supplier is normally the publisher because that's who a retailer pays and makes a contract with. Even if you work with a file distribution service they typically send the book file to the retailer at your request. You choose to place the book on the retailer site and they pay you for any sales of it. A good rule of thumb would be: If the money comes direct to you from a retailer then you are the supplier within an ONIX feed. If a retailer needs to contact you to sell a book then you absolutely are the supplier of the book.
"Why do I need to make ONIX?"
ONIX metadata isn't used by consumers directly — an ONIX record is used by businesses to create the book records that they display on-line or sell to clients. Letting consumers know which companies sells your book is part of your marketing. It is true that ONIX metadata is used by services supporting book discovery, like 49th Shelf or in a broad sense BookNet Canada's BiblioShare. ONIX metadata does not support which libraries hold this book or which retailers, have it in stock or sell it. ONIX for Books supports what books are available to other businesses and hopefully contain the detail they need to support a sale. If a discovery site wants to show actual book availability they use programming to check retailers to ensure they carry a book. There are apps and services that search out the lowest price for consumers. Apps need metadata and they may source that data from BiblioShare among other sources, they won't use the Supplier Name to inform anyone that it's available at Amazon.
What if a retailer has an "exclusive" right to sell a specific product - this ISBN is only available from that one retailer or chain? Then you need to use something called Sales Rights Restriction within ONIX to say the rights on this ISBN are limited to the retailer. Generally though Sales Rights Restriction aren't seen much in ONIX data because the easy solution would be that the publisher just doesn't send data on that ISBN to anyone other than the exclusive contract. They would normally have another edition available for general sales and distribute data on that book. But even if the book is only available from the one retailer, and no where else, it remains the publisher's problem to marketing the book, though the retailer may make commitments based on an exclusive sale. That's not really some metadata can sort out and ONIX is best used to support supply chains and typical use cases shared by multiple businesses. If it also accommodates other use cases the question would be one of distribution: Who will accept this metadata and why?