Why should they have to? Where they driven to this because sellers have a no refund policy? Since the buyer can't examine the item before purchase then the seller must have a no questions asked return policy. If they don't, even if that fact is clearly advertised, then they open themselves up to problems. Have a return policy and this won't happen.
As an honest buyer, I don't want to be held hostage for my feedback. What if I genuinly need to leave less than positive feedback? Then I get trashed even though I fulfilled all of my responsibility by having payed. The seller wants to get even. That isn't right. As for those scammers you mentioned, I think that falls under the heading of petty crime rather than a legitimate buyer/seller relatioship.
Your points have some validity, but I think that on balance this policy is right.
We had a 100% plus shipping return policy. But, there being no consequences caused people not to be careful to research what they were buying or just to abuse the return policy. Some people were also just buying things just to look at them and then returning them. It really adds up when someone buys 20 different items on 20 different occasions and returns every single item. The losses from lost shipping fees overwhelmed any profits. Then we changed to 100% refund except shipping. Ebay will also remove your power seller status if you fall below 98% positive feedback. We have had a huge increase in buyers that bid on, and win many items and then contact you and threaten to leave negative feedback on each item unless you reduce the invoice to a rediculously low price. We have managed to maintain a 99.8% positive feedback and probably 95% of sales are without a single problem. It is just that 5% that makes you shake your head.
I should also add that since there is no way for sellers to leave negative feedback and all buyers have 100% positive feedback, a good feedback score does not mean anything at all. Sellers have to be suspicious of everyone and buyers have no incentive to be civil or honest when dealing with sellers. Amazon also has no way for sellers to leave feedback, but it does not seem to have the really bad characters that ebay has attracted.