I’m not sure to which part of the United States Code you’re referring with that definition, and we all know that our Congress-critters have long done a poor job in terms of clarifying conflicting meanings in this or that Public Law - but 15 USC Chapter 110 §8402 specifically reads thus:
In this section:
(1) Initial merchant
The term “initial merchant” means a person that has obtained a consumer’s billing information directly from the consumer through an Internet transaction initiated by the consumer.
(2) Post-transaction third party seller
The term “post-transaction third party seller” means a person that—
(A) sells, or offers for sale, any good or service on the Internet;
(B) solicits the purchase of such goods or services on the Internet through an initial merchant after the consumer has initiated a transaction with the initial merchant; and
(C ) is not—
(i) the initial merchant;
(ii) a subsidiary or corporate affiliate of the initial merchant; or
(iii) a successor of an entity described in clause (i) or (ii).
I’m fairly certain that our friend is Sawle Mill is correct in speculating that it is this provision which has led Amazon to launch this initiative.