Separately, there is a 15% transaction fee for any commodities (again, only if they sell), which are items that stack such as gems and gold. We take a percentage due to the way commodity sales work. You can put up 50,000 gold for sale, and because people can buy portions of commodity auctions, one person may buy 10,000, another may buy 30,000 and maybe nobody buys the remaining 10,000. Instead of charging $1 per transaction, which certainly wouldn't be fair to the seller in the case of commodities as they have no control over how many different people may buy from their auction, we go with a percentage. If that original 50,000 goes up for say $5, there would be a $0.45 fee on the 30,000, and a $0.15 fee for the 10,000, totaling a $0.60 fee for the sales that occurred.
Just to be triply clear, these fees only happen if the item sells. Putting an item or stack of items up to see if it/they will sell doesn't have any inherent risks, and is a good way to get some Battle.net Balance built up. You can sell items you don't want, and then get something to spend on something you do want.