This entire chapter is, by its nature, Cameroon-specific: the roles of transitaire and commissionnaire en douane agree, the CEMAC accreditation system, the ECTN/BESC requirement enforced by the Cameroon National Shippers' Council, and the GUCE single window are all part of how Cameroon's import and export chain is organised. If you already worked through Chapter 12 on documentation, you will recognise the ECTN and pre-shipment inspection requirements from there; this chapter is about who actually handles them on your behalf, and how to be sure that person is who they claim to be.
Your country: find the equivalent licensed customs broker or freight forwarder category in your own country, and the official register or licensing body you can use to confirm an agent's credentials before hiring them.