The buyer is an entity ordering a connectivity service or component of a service (either retail or wholesale)
Specifically for a wholesale service, a buyer of Carrier Ethernet wholesale access services is a Service Provider that needs access to a Carrier Ethernet UNI that is not on its own network, and is willing to buy a Carrier Ethernet service from a CEN Operator that will connect between that offnet UNI and the service provider's own ENNI.
A global Tier 1 service provider called "GlobalSP" has an ENNI located in Cape Town, South Africa. GlobalSP needs to provide an E-Line service between a customer site in Chicago and another customer site in Johannesburg. A regional operator called SouthAfricaSP is able to provide an E-Access service between the Cape Town ENNI and a UNI located in the Johannesburg district where the customer site is located.