Because eBay being a the middleman in this sale, they require a minimal fee. In the making of your profile, you were provided the Terms and Condition further explaining such fees. This is, however, the way they make their money. Although, depending on the item, category, and asking price depends on the fee taken by eBay.

Though, the fixed percentage subtracted from your amount made by the sale is 10%. Not bad at all if you are in desperate need of the item finally being moved out of the house, or even just for quick funds. Also, the max amount they will take from you is $250, even if that it less that 10% of the sold price.