How to motivate the user to pay for the service provided?

Question

Time Management

In order to gain experience in creating web applications and at the same time do something useful, the idea of ​​​​a project was born, the essence of which in a nutshell: there are professional users and novice / consumer users. professionals can help beginners in solving their problems, for this, I would like them to receive financial gratitude.
So, how best to do so that novice users do not deceive professionals, only "thank you" for the work done.
I would like not to directly intervene in monetary relations and avoid prepayment.
Prepayment is bad for the following:
1. If there is no adequate professional who can solve the problem, and then the money is paid.
2. Possible distrust of the user - “I will transfer money, but they will go nowhere and they won’t help me”
I would like to make payment after the fact, after reaching the goal.
I see the following ways to make it a little safer:
1. When registering, the user enters a valid phone number, which receives an SMS confirmation
2. A shame board with the publication of those who were helped, but they, in turn, did not thank.

Answer 1

the first one is free + the rest according to the “safe deal” pattern.

Answer 2

if you have newbies with their newbie questions, then they will thank $ 1 for help :)
It’s easy to do, the usual rating, only when a newbie adds a rating from his account to a pro’s account, 1 USD also goes. Also, mark newbies with a label - gives XX / month rating, total gave XXX. The truth is not so obvious, but more cunning, but the point is that the professionals would see what it costs to help him :).
It is also desirable to have a normal rating, so that pros can mark -1 in the profile of a "bad" newbie :)
This is the simplest system, but it is designed for a regular support forum. If you need a more substantial reward, then there are other implementation options.