I'm sorry to be the one to tell you this but, without tracking, there is no possible way for you to prove your seller has received it.
You are out of a product, and the money for it.
Did the seller tell you to return it? He should have also told you that it needed to be tracked. Furthermore, if you went through the official ebay means of Returning an Item Not as Described, the seller should have been required to fund that cost of return shipping.
In other words, you are sunk.
How much did you lose? Maybe if you call ebay Customer Service, they can give you more hopeful advice,
