That being said.
Most of the time you do NOT want to talk to an agent.
Did your item not arrive?
Go to the Resolution Centre and open an Item Not Received Dispute.
If the seller cannot prove delivery you will be refunded.
Was your item quite different from what you bought?
Go to the Resolution Centre and open a Not As Described Dispute.
You will be told to return the unwanted item to the seller and your payment will be refunded. In some cases, the seller may pay the return postage, but don't count on this.
Did the seller leave you rude feedback disguised as positive?
That reflects on him more than on you, and most sellers never look at the fb of their buyers. But this is one case where you can call eBay and have the fb removed.