That is the practice known as "gouging", very much discouraged by eBay, but when you ask eBay directly what they are going to do about it, they go a bit shifty, I'm afraid.
I find the best policy is to just refuse to pay, and offer the correct postage and reasonable handling charges.
I don't count being negatived by an eBay seller as the worst catastrophe that could possibly befall me! But there again, it just depends on how badly you want the particular item.
Cheers, Steve