Build system where two users provide eachother a service that is performed outside the system, but needs to be verified

Hi All,

I'm stuck with a (conceptual) problem and am curious whether some of you can provide some feedback.

Objective: Build a system (sort of a marketplace) where people come who have a shared problem. In this case, it concerns the completion of surveys for dissertations; everybody needs to have a number of respondents and spams facebook etc to acquire these. My idea is to put them in a 'marketplace' where people can fill in other surveys in exchange for having their own survey filled in.

I'm looking for a good idea to build this system. How can the system control for the fact that two people have filled in each other's surveys? Or would it be better not to directly link two surveys, but have people earn 'credit' by filling in surveys which in turn can be exchanged for having their own survey filled in.

The main issue -i believe- lies in the verification, how can the task that is performed outside the system be verified in a credible and user friendly way?

Any thoughts/ideas would be greatly appreciated!

Please let me know if anything needs more clarification; its a bit of a rough draft.

----------USE CASE [draft]-----------

User A registers and provides link to survey

User B registers and provides link to survey

User A conducts survey of User B on external Platform

User A notifies the system that the survey is conducted

User B conducts survey of User A on external Platform

User B notifies the system that the survey is conducted

System has to ensure sure both users have completed task.

System notifies both users about completed tasks