I disagree. In my mind it helps keep a time stamp and history of who is interested and when they contacted the OP.
True, but the seller has no obligation to follow it and nothing is done if the time stamp was not followed. Only the seller really knows the order of the PM's received. I have seen this become an argument in exchange threads as well. So I agree with the comment that the "PM sent" serves no real purpose. Eliminate conversations in the exchange threads- eliminate the majority of all unfriendly interactions...