I edited my own post. It turns out that that guy starting up the mining co. had posted two identical threads. One in econ > marketplace, and one in econ > marketplace > selling.
When I checked in the morning I looked in the 2nd thread, not realizing there were two of them, so I thought my posts had been edited. I still do think it's a little iffy for the mod to be running an exchange, but until there is evidence of him deleting posts or something else nefarious I wouldn't worry too much about it.