Since when is the buyers obligation completed just because they have paid?

What other obligation does a buyer have? If you buy something in a shop your ONLY obligation is to pay for the item. Well, I suppose if you want to be pedantic, you also have an obligation not to wreck the shop on the way out.

How is that different to a buyer on eBay. You are saying that the buyer must be kind and considerate and leave me nice feedback or I'm not going to leave him any?

I know that feedback is volutary and that there is no real OBLIGATION for either party to leave it, but, when all is said and done, the whole theory behind feedback is to let other users know the reputation of  members / potential trading partners.

I am all in favour of buyers and sellers being able to leave bad feedback if the situation warrants it, but, to WITHHOLD good feedback from a good buyer just because he hasn't left you the seller good feedback, is BLACKMAIL & WRONG!