I don't think eBay is really an aggregation service; it's a platform on which people directly sell items. An aggregation service on the other hand goes out and finds shops already selling items through other platforms and provides one place where you can search all these shops. They would then presumably collect some sort of commission on sales.
eBay is not forwarding you to third-party websites in order to purchase products. They are listing products directly. The company the author was creating forwarded you to someone else. In return the store paid a monthly fee. Essentially this company was targeted lead-gen, and stores paid a fee for them to do the work and drive traffic.