My feeling is that alot of people are just lazy and don't bother to respond b/c it's already sold or something.
Not very considerate IMO, it's only takes 5 secs to hit reply and type "Sorry, it's been sold".
That or people may not believe you are a real, interested person?
Not implying you specifically may not do this, or are a yahoo, but I make it a point to use proper grammar and punctuation, and even write a couple of sentences to (hopefully) give the impression I'm an intelligent person and actually care about this transaction.
Personally I rarely respond to emails w/ half-assed grammar, incomplete sentences etc.
I've found a high correlation between the time people spend writing their email and how likely the transaction is to follow through.
