an obligation to:
1. Pay for the item timely.
2. Provide accurate shipping information.
3. Communicate when the item is received. (especially international as it is difficult to track)
4. Communicate any dissatisfaction with the item and give the seller an oportunity to rectify any issues they may have.

I the above is done, the sale is complete and then feedback should be left by the seller. If the above is done, then I leave feedback. IMHO a sale is a completed transaction not a payment until the merchadise has changed hands satisfactorly.

In the past sellers had the advantage as they could leave negative feedback for the buyer and would intimidate a buyer from leaving negative feedback knowing there feedback could also be negatively effected but without that option, what difference does it make to wait until the sale is complete?
I do not base my feedback on the feedback left for me but on the above criteria. So even if a buyer wasn't satisfied but communicated such and gave me the oportunity to fix it, they deserve great feedback but if someone paid me fast but left poor feedback without giving me a chance to fix it, they would get a positive for paying fast but that is not a buyer that I would "recommend highly" to another seller.
This is all just my opinion, take it for what it is worth.