There's always going to be bad apples in every bunch, but I would say that the vast majority of sellers do not hold buyer's to feedback ransom. Personally I leave feedback after it's left for me because I'm interested to know if the buyer is happy once they receive their item. My job as a seller does not finish when I get paid and send the item out. I will generally leave feedback after about a month or so if I remember, because I would assume after that length of time the buyer would have either received their item, or contacted me to let me know there was a problem.

However, the complete opposite is true for buyers holding sellers to feedback ransom. Buyers are the only ones able to leave negs, and have been known to threaten it if a seller doesn't bow to their terms. It's completely one sided, the only way a seller can express a negative experience is to leave no feedback whatsoever. Meaning they can't communicate to the ebay community of a problem buyer. This is a very big flaw with the system. Problem sellers are easily spotted, problem buyers are not.

If you try to sell something malcolm, you will get a more balanced view and understand why some sellers do what they do. Feedback is completely optional. A buyer can choose to not leave any, so can a seller. That is the only balanced part of the whole system.