The investigation is also looking at the role of data in the selection of Amazon's "Buy Box" winners which appear on the right of product pages saying "Buy now" or "Add to basket" and drive about 80% of sales.
However, the EU's competition enforcer is now considering splitting the two issues to focus on one, making it easier to move forward with the case, one of the people told Reuters.
"The data case seems weaker," the person said, adding that no final decision had been made on whether to split the case, nor which angle to focus on first.
The European Commission declined to comment.