As a buyer and seller on ebay, I can confirm the following:
Only buyers can leave negative feedback. Sellers only option is to not leave feedback or to leave positive feedback. So as a buyer, if your transaction is canceled post sale by the seller, you have every right to leave negative feedback about this to warn others. Similarly, if you are a seller and the buyer doesn't pay, file an unpaid item case and get a strike on the buyer to limit their ability to bid/buy future items. It is a two way street and if you don't fulfill your buyer/seller duty, it can and should have consequences.
On the fee side, ebay charges 10% of transaction price (item+shipping). Paypal charges 2.9% of transaction price (item+shipping) + $0.30. These are for domestic transactions. International can be different. To get lower pricing, you would basically have to be an ebay store and sell many items per month, allow returns, etc. As a general rule, assume fees will be 15% of the total transaction cost (item+shipping).