A point most dont consider here is that, from previous stated dev replies, the attempt to manipulate prices is enough to get banned. You dont actually need to change an items price at all. However, the other way around, people who think every change in price has manipulation as cause are obviously wrong too.
So, if there are people who strategically post threads to lower prices they deserve an ban. It doesnt matter if the price did change or not, the attempt counts. Just as in ign selling on forums.