10xp is not a feedback worthy trade. Report them.
Obviously we can't do anything if two people give feedback to each other then say "No, there was a legit trade." nor is feedback a big enough deal for me to invest a lot of time into making sure it was a legit trade.
IMO, here are what my thoughts are on feedback:
Must have a thread
Must have been a legitimate service (not "he got me 10xp!")
Must have evidence of a service
That's it.
So the Optional link to thread box should be filled 90% of the time.
Of course, there are going to be situations where people do trades without threads. I do it quite often. If the user is doing a regular service, he should have a thread. if he has 5 feedback with no threads, who cares. If he has 50, he should have a thread. This would also enforce the post requirements for account sales, etc. since they have to post on the thread to make it a legitimate service.
If there's no posts, and no evidence of a service on osbot, then there should be no feedback.