I take it as first come-first serve and I do not contact the seller after someone sends a pm sent message(especially on KLOV) To me it is about respect.
I dislike bidding wars and sellers that entertain in such practices so I figure if a guy contacted a seller first-he or she deserves first right of refusal. That being said I usually wait for the seller to reply several days later that the game is still available. I have had people say pm sent-kick tires and then I literally have to say-'still available' days later.
I do think it is an acceptable practice and it helps form a line of interested parties. I hear stories about games being sold minutes/hours after an inquiry is made and I dislike this practice. If someone contacts me for a game-I work with the first person and go down the line from there. Well I had someone contact me five minutes after you offering me more money-pathetic