Server assembly is not part of the job description of most system administrators.
The basic skill set required for server and computer assembly is partly what you would learn in kindergarten, square peg goes in square hole, some common sense and partly what you would find in an (introductory) vocational training course in micro-electronics, electrical engineering or similar at a local community college.
My impressions of the staff working in independent computer shops that assemble PC's (and sometimes servers) from commodity components are (stereo-) typically either computer enthusiast without much formal education, or have some form of the aforementioned vocational training. They are not system administrators.
Components would be selected based on the price, reviews and performance of the individual components, instead of their contribution to the complete end-result. My expectations for quality control would be inconsistent and testing limited to a successful OS install. A big reason not to consider those systems for the larger scale deployments I associate with professional deployments.
Most of the large brands have your complete systems assembled on demand from a limited pre-selected and validated set of components in their own factories or by their local distribution partners. Most vendors will organize some level of training and certification for the staff those facilities, but that training will be specific to their product lines and only open to those staff and not the general public. Those staff are either semi-skilled assembly line workers and/or have a background in micro electronics, electrical engineering etc.
They'll be provided with quality control tools to verify that assembled system is build correctly and fully functional.
The actual design of the server systems and their options, the selection and design of components etc. that is done by more highly skilled engineers in the main product development labs of the large brand name vendors. Individual components should be tested extensively as well as integrated testing with the assembled systems. Relevant training for that would either be an university engineering degree or PhD specialized in electronics or a CS degree for staff writing the embedded software, drivers etc. As a small shop or individual you can't and shouldn't desire to compete with that.