Outside of Custom we find sellers who work with a custom product but they do it in a way that I imagine gives them more problems than anything else . I have looked at several cases that use the keyword "custom" and I found that almost all of them ask the user to perform some additional action that is not at all simple.
For example, purchasing a customizable item, paying for it, and then having to the advantages of line database send the image you want to use via email or add text within the gift options... all impractical options.
Simply, Amazon is not ready for personalization these days.
This is not to say that it never will be, in fact it is actually moving in that direction, but at the moment it is not prepared to offer this functionality, if only because of the cumbersome user interface and procedures.
What does this mean? That for now, product customization, along with customer service (in the pre-sales phase), can be a winning card to play for eCommerce that want to stand out from Amazon.