I personally don't really care too much what a reviewer personally decides to do with a free or purchased review item.
Trading a (possibly free) review sample for a higher value item after publishing a review that claims the IEMs reviewed outperform other headphones similar in value to the item that he is trying to obtain via trade does give me strong pause. Writing an extremely positive review and highly recommending it prior to immediately trying to get rid of it does indeed seem to be a quite interesting choice of action that appears a bit contradictory to me.
Basically, reviewers can do whatever they want with reviewed gear,.... I would personally be wary of the content of the review...