The barrier to entry would work the same way it does for any profession.
To make it into a profession there would have to be:
Some sort of education requirements,
Some experience and guidance (I.e. working under a professional for a year or two),
An examination showing proficiency,
A list of rules that members must follow or face disciplinary actions,
A discipline process for those that do break the rules,
Continuing education requirements to make sure that members stay up to date,
And an oversight board to ensure that everything is being followed and victims have a place to file grievances.
(This list is by far not all inclusive.)
There’s nothing that stops friends and family from suggesting shidduchim just like there’s nothing stopping you from getting friends or family to fix your car when it breaks down. You’re taking a risk though as you may void the warranty if a non-certified mechanic works on your car.