What other obligation does a buyer have? If you buy something in a shop your ONLY obligation is to pay for the item.

As a seller, then, my only obligation is to post the item. Once I've done that, the buyer should leave me feedback immediately - who cares if they haven't actually received it or anything, it's not like there are a bunch of different things that could go wrong and be handled by me in an incompetent, ineffective or just downright rude manner.

Feedback isn't about obligations, it's not a part of a transaction, it's an assessment of one. If a seller doesn't want to assess the entire transaction until they have more knowledge of it, that is their perogative and the word "should" doesn't belong in a sentence about feedback, unless that sentence is "every member should work out the system that suits them". Just because you have worked out your way, doesn't mean you can turn that into an expectation of others.