Can some one help me please.

I rececently was the winning bidder on an item on ebay and happily paid the money ($335.00), subsequently the seller contacted me and said that they had noticed a flaw in the item and were not comfortable selling it to me would I like a refund.   I said yes and we agreed on the refund.   A few days later I received a mesage from ebay saying that the refund had been issued and quoted my bank details (which were correct).    That was three weeks ago and the money is still not in my account.   I contacted the seller who says that I need to contact ebay, but who do I contact, or even how do I make contact?   Thanking the brains trust.