I'm pretty sure he's referring to when you have multiple items listed in the TP and someone PM's an offer with no description whatsoever of which item they are making an offer on.

The other one that gets me is a "I am interested in your (insert item here)" with nothing else in the message.

OK I get you're interested but either say you'll take it, or make an offer, or something. If it's something I have another offer on, even if this was first, I go to the next one. IMO Bad communication or wishy washy is just a person waiting to flake out on you.