If I leave negative feedback, can the seller do the same to me?
In short, the answer is no. Since May 2008
it is no longer possible for a seller to leave a negative or a neutral feedback for a buyer. If you feel they deserve bad feedback, then they can't retaliate in kind. If you also sell on eBay, it's a good idea to get yourself a buying ID
to protect your account from retaliation.
Of course it's always best to contact the seller first to give them chance to resolve your issue before leaving bad feedback for them. If they're a powerseller, you have to wait 7 days from the purchase date before negative feedback can be left.