Double charge or not? End-user and Provider?

Imagine you would like to build a directory where users would save time finding providers which sell complicated services/products.
Do you bill:

• the user because it saves them time to find the product using your platform? (monthly subscription?)
• the provider because you are helping them selling their stuff (a percentage?)
• or both because everybody wins?

I am sure the answer is : "It depends...".

So here is some context:
Engineers (working for companies) need to run some very specific tests to confirm what they think is right.
Finding the companies running these tests is slow (find through google a few that can do what you want and get quotes). It's time consuming (1/2 to 3 days).
Then on the other side you have several companies offering all these tests but they all have a bunch of tests explained differently or requires sales contact to understand your needs.
My goal is to connect them both so everybody wins. It's definitely a niche with a few challenges like categorising these tests properly to allow end-users to find stuff quickly (if we can get the providers catalogs!:).

They are several business out there connecting people with a directory/listing (how do you call these type of services BTW?)
But they all have different models. AirBNB seems to charge both sides. Amazon, only the provider. Some even seems to live on ads, etc

How do you come up with the right "billing formula". I am not even talking pricing :) Just who to bill? :)

Reminder: In my case the "end-users" are employees so it's B2B, not retail. Also the tests are not cheap we are talking several thousands dollars

Any inputs, links to urls are welcome

Cheers
Thomas