I think the only 100% proof in such cases would be actually to engage in such a discussion / transaction and post here the screenshots.
Be very careful there, "sting operations" are frowned upon and sometimes lead to negative feedback on the person carrying it out.
I still think merit sales aren't really a problem. It's expensive, we can't stop all abuse, it's a tiny fraction of the total Merit economy, but most of all: it's very obvious when a bad posts receives Merit. Any campaign manager who does his job could easily spot that.
jvdp already has negative feedback for cheating a campaign.