PM sent.
I never understand why people post stuff like this. It's utterly redundant. I see it on Facebook all the time too. Literally no need to post publicly that you've PMd someone as they'll get your message (and if it'd to do with sigspam then you wont get paid the post on bitmixer because it doesn't meet the minimum requirements so what the point?).
The reason can be to warn OP. Sometimes people starts a thread buying/selling sth. Then they get some pms but dont return senders even if they are online. So the purpose maybe publicly notify or warn.