You might be 99% sure, in your mind, but you have absolutely no EVIDENCE for criminal prosecution.

If you had a record of YOUR serial number on a receipt or in a photograph, THEN you might have evidence for criminal prosecution if your ex's serial number is the same.

Finding out the store where HIS serial number was purchased is meaningless and no DA is going to touch it without EVIDENCE that proves beyond reasonable doubt that your ex stole it, especially if you are talking a couple of hundred dollars or so.

However, the standard of proof in small claims court is less rigorous and only requires a preponderance of the evidence. Frankly, I don't think you even have that but small claims court is cheap and informal. Up to you if you want to take a shot at it.

Or, you can just get over it and stay over it.