I think sending a message is a great way to start establishing communications, but just as you have stated some sellers don't seem to respond to a message in a timely manner. Don't understand why, They just don't...
Messages are fine and informational for both the buyer and seller, but its been my experience that a message is just that.... a message.
I have found most people who are serious about buying or selling something will call you as soon as they see your ad or receive you phone call.