You don't need to call anyone, you open a case, but depending on what the item was, and whether it is a business seller, does not fit is classed as a change of mind return, and a private seller does not have to acctept returns.
If you have sent back without using the proper ebay system, then you will probably find it difficult getting a refund.
Does not fit usually refers to clothing but so long as seller sent exactly what you ordered, only a business seller has to accept this sort of return, and you the buyer has to pay return postage.
It really is time you read the ebay 30 day money back guarantee, it is on every listing, right side under the payment options.