@commercee can use eBay only by configuring the categories to show eBay products.
Alternatively, I was able to enable access to the API for a new account by sending an email and showing them a blog, basically this way:
1) Open an Amazon Associate account
2) E-mail them within your Amazon Associate account to request the product's advertising API. Provide a blog with recent content.
3) Once Amazon accepts and approves the account, go to "Tools -> Product Advertising API" and click to generate the keys
4) Keep them in a safe place
Email for (2) is:
I would like to use the product's advertising API to automatically update products and prices on my blog, instead of doing it manually.
Can I request that the product's advertising API be enabled in my account?
I would recommend using a blog to get approval, which has minimal content and ads. This only guarantees that they will approve it (different Amazon agents do different things!)
I hope that helps