Some do it one way, and some the other. If you want to leave feedback first or second thats your decision.
What happens if an item goes missing in the post? Should the seller have left the buyer positive feedback as soon as they posted it and then the buyer leaves the seller negative feedback because they did not receive it? Or should the seller investigate the missing item and get your money back for you? The transaction is not complete until the buyer has received the item and is happy with it.
The way I see it, its a matter of courtesey. If you know your buyer has received the item and they let you know, then that is good manners. If they received it in poor condition or not at all, I want to know so I can sort it out.