Well, in intermediary could be the company which delivers your goods, such as DHL or TNT, or the shop which stocks you goods. DHL could be an intermediary for BPP study texts.
Complementors are organisations that increase the demand for your goods (competitors reduce demand for your goods). For example, ads pop up on this website advertising, say, BPP material, so OT is a complementor for BPP. The availability of last minute hotel vacancies on the internet could be a complementor for the airline industry