Well ideally everyone should gain from this 4-way arrangement: the producer making a transaction they would otherwise not have made, the consumer covering their

Well ideally everyone should gain from this 4-way arrangement: the producer making a transaction they would otherwise not have made, the consumer covering their need efficiently/wasting less time on something/consumer surplus, the site operator participating in allowing the advertiser to gather the data necessary to make the recommendation and the advertisier getting a commission for for coordinating all that. Of course in the real world we get more of a mexican standoff than enlightened tradepartners coming mutually beneficial agreements.