Did you open an official return case with eBay or did you just send it back without a case? I assume you opened a case if the seller paid for the return shipping. If you do have a case go to it and see what the refund status says. If you don't have one and it's still possible to open one; do so with the correct reason for the return and enter the tracking number into it to get the process started.
I'm sure someone else who knows more about this issue will come along and provide more answers.
