These new "middleman sellers" as you put it are not allowed to post here if they do not have inventory. I have personally talked to a few members and issued warnings about this practice, although I'm not sure if it's the members you mentioned. Once someone is warned, if they continue they will then be banned.
However we reply on member reports heavily. Local and Global mods will obviously report things and escalate issues when they see them, but most of the time we reply on member reports.
If we don't see it and it doesn't get reported, it'll go unpunished.
You mentioned a few members in this thread, so I'll review their sale threads and contact them for more information. If found to be offering a "middleman" type services, they will either be warned and have their threads removed or banned if they have already been warned.