XenForo does have a trading rep plugin/addon/mod.
https://xenforo.com/community/resources/xencentral-trading-system.448/
However, I'm not to sure how useful it may be around here.
The image below is of a user on one of my other forums. It is of no way a "rep system" in itself as the ratings come from all of their posts across the forums. However, you could turn it into a system for reputation on trading. Would require a bit of work and custom edits, but do able.
At this very point in our stage of development, is it a requirement though? At the moment if you where to send those 100 rebellions over to a user, and the code he did provide ended up being a fake, we can easily reverse the transaction. Even issue a warning or possibly a ban depending on the severity.
Knowing how trustworthy is from the beginning though, could prevent the entire thing from happening. So I will look into it