Unless it's a buyer who I know (ie: one who's purchased from me often enough that I recognize their buyer ID), I generally don't leave feedback after I've received feedback from the buyer.

Main reason is that if I receive feedback from the buyer, I know they've received the item and are satisfied, and my leaving feedback therefore concludes the transaction for us both. 

As for the whole "leaving feedback for the buyer lets them know that the item's been sent!" reasoning, that's what the "Mark As Sent" bit is there for - the buyer will see that the item has been marked as "posted" in their Purchase History.