Some of the sellers are just out to scam money on shipping. I buy alot of books on ebay. If you look through book auctions you wills see some crazy deals for books like buy it now for .01 if you look close at the fine print they will charge insane amounts for shippping like 13$/pound

. Its sellers like this that are out to dupe people then intimidating them into paying with the threat of a bad feedback. Somone with less than 100 feedbacks can have thier feedback percentage severly damaged with one feedback, while the scamer has so many feedbacks one bad one wont change one tenth of a percent.
That said, there are some legitimate reasons for high shipping costs on ebay. #1 ebay's final value fee is way more than it should be. #2 If you accept paypal payments there is another portion of the final value gone. #3 aside from those fees you had to pay to list the item in the first place (if you add this up just 1-3 are already half of the 5$ shipping fee for a 9.99$ item and the seller hasnt even got it in the mail yet) #4 bubble mailers and other packing materials arent free #5 postal fee's are nearly as bad as the ebay raping. Then you have to think that for some of these powersellers they are actually running an ebay buissiness where they are paying people to work for them doing things like packaging and shipping, if they dont charge some kind of handleing fee added to the shipping fee they are cutting into the profit from the actual item which is usually already cut to absolute minimum anyway just so they can compete with prices.Of course you could get it cheaper if you go to walmart and buy it, you are paying extra to be able to shop in your underware.
A good way to tell the two types of sellers apart is what they charge for combined shipping. A good seller will usually charge very little for anything past the first item (just enough to cover the addition ebay+paypal fees). scamers will not list the combined shipping rates and typically will not respond to questions about them.
I understand the argument for leaving feedback after payment is made. Some sellers do that. But overall, its just inviting some people to try to ruin your ebay seller reputation over little things or nothing at all, just for spite. Ive done quite a bit of selling on ebay and have had just about every scam tried on me at one time or another. In the end, holding feedback until it is left by the buyer first is one of the only protections a legit seller has. Paypal almost always sides with the buyer, because if word goes around that buyers are getting screwed people stop buying and ebay looses money.