A retail supplier or catalog house that sells professional audio equipment does not design or engineer sound systems, and in most cases they’re not qualified to do either. All but a few of the largest do not employ seasoned live sound system experts. Very few, in fact, have staff with significant experience in live sound system design or operation.
Staff members are primarily part-time musicians or home studio owners who are there primarily to supplement their incomes. And musicians and home studio people are rarely qualified to provide sound reinforcement design advice, nor do they usually have a grasp of architectural accommodation, nor do they possess the necessary skills in installation and related safety issues, nor electrical systems and related issues, nor the physics of electroacoustics.
Further, they’re not familiar with the National Electrical Code (NEC) that must be adhered to, both for inspections and insurance purposes.
Usually, they can’t properly employ room and system modeling (computer-assisted prediction), and they’re not trained or invested in test and measurement skills/equipment that help truly optimize a sound system.
They’re usually not members of any pro audio and/or acoustics trade organization, and they’re also prone to dismiss many significant issues as “not that complicated.” (And if you hear someone say this, run away screaming immediately!)
Note that I am saying most - not all - fit these criteria. Anyone still bold enough to tread down this path should at least understand the right questions to ask.