I feel like this makes very little sense, because a purchase is a trade - one resource (currency) for another (the product/service). If the product has no value, there is no reason to engage in the trade. This can only exist for the purpose of wealth transfer from the operator of the shopping robot to the seller of the useless products, or as a facade of some sort.

You are right.

The novel is a bit of a dark comedy sci-fi. And even though many details are surprisingly accurate (for a story written a decade ago) robots buying crap produced by robots is IMO meant to illustrate an absurd racket to inflate demand.