Finding a qualified developer or an agency these days is a rough endeavor.

* Verifying output and credentials is nearly impossible for a non-technical business owner.
* Assessing product quality upfront is challenging. A custom build will be different even if you send the same assignment to 10 different companies.
* Setting the right KPIs upfront is tough. Even if you establish some traffic/data volume limits, you'll require both time and expertise in order to validate that.

Determining quality by price won't work every time, either. There are cheap developers who produce outstanding work and expensive consultants who can't deliver.

Try to break the process down into separate components. For example:

* A mockup/wireframe/sketch of your website.
* A sitemap of all pages.
* A use case diagram defining all roles and how they interact with the platform.
* A detailed scope of work defining all project requirements, the key features that the platform needs and sample user stories that should validate upon delivery.

A good starting point is picking the right technical stack for your platform. Could be a CMS such as WordPress or Drupal, a framework like Laravel, Ruby on Rails, Django, or a completely custom platform. All of these have pros and cons and you can find a technical expert in your network who can assist once you have the initial brief ready.

Certain platforms like WordPress have designated marketplaces for experts. TopTal is also a good place to find great talent. Looking up online may uncover experts and companies profiling in similar industries. Word of mouth works great if you ask around in your network.