I've a message to Mr. Dell (also wanting to buy) which is still in my 'outbox'. I assumed that they move to the 'sentbox' when they are opened by the recipient?
If they are still in your outbox they have not been read by the recipient.
Resend so they get another email reminder. To be fair to Mr Dell I suppose he gets a fair few PM's over a given period so he might miss the odd one or two