With only 52 feedbacks, you are dealing with a novice. You may be able to reason with him that you understand his skepticism, and that you would probably feel the same way if you were in his shoes, HOWEVER, you are 100% certain that you did not receive a 107mm spindle, and would appreciate it if they would ship the correct spindle in exchange for the 115mm one. Also, casually mention you are a member of the Bike Forum with thousands of members who are monitoring the outcome of this transaction.

If that doesn't work, then you can always submit a request for arbitration through ebay, through which you can state your case, and through which he will be forced to respond. The last resort is negative feedback.