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.

No! That is not what I am saying. As in the shop scenario, once the item has reached the buyers hands and he has checked that it is what he ordered - THEN he leaves feedback.

If something goes wrong in the transaction ie you sent the wrong item - of course it's STILL YOUR obligation.

If you sent the right thing and AP broke it then the obligation is for one of you to fill in a lost item claim - probably best for that to be you - BUT it's STILL not time for the buyer to leave feedback, unless you want a NEG of course which would be a little unfair.

If  feedback is NOT given by buyers until / unless the buyer leaves a positive for the seller - it is quite feasible for an excellent buyer, who has diligently purchased MANY items over a number of years - to still have zero feedback. If that buyer then tries to buy an item which a seller has defined as 'no zero feedback bids accepted' - BOTH the seller and the buyer LOSE OUT on what could have been a straight forward transaction on the parts of both members.

As a few have said, it's eBay's site - they introduced the theory of feedback and it being a guide to a member's selling / buying reputation - eBay made feedback voluntary - if people are not going to use it as it should be then why not delete it altogether?