You don't need to contact eBay. If the item you received (a spare part for a vehicle, I presume?) does not match the description of the item you bought you can use eBy's money back guarantee.
Until recently I would have warned you that you will probably have to pay the return postage to the US. Now eBay seems to be making the seller responsible for this, as they do for MBG returns within the UK (except that eBay can't supply prepaid return labels for overseas returns).
It's all a bit uncertan as eBay doesn't seem to be consistent in it's handling of this. Ask here again if you run into problems. By the way, if VAT or import duty was payable you will have to reclaim this yourself from HMRC.
