I'm assuming you shipped with USPS. Do you have a copy of your label receipt showing the service you paid for? That will also show the billable weight.
Contrary to popular belief, USPS does get package weights at each scan along the sorting stations. This information is not part of the external USPS website tracking that we can see as customers. You would have to get the internal USPS tracking, that they are not required to give, but may if you ask nicely. Compare what you sent, versus what you received. Assuming the buyer didn't stack the package with magazines or something to give a comparable weight, and assuming USPS will provide the information I touched upon above, you should easily be able to appeal this is you do some legwork.
Well.... nothing is easy with eBay but still try and get that USPS information (nothing says you can''t ask at another post office if the first turns you down), file a police report (even if it does nothing the case number will help with eBay), file an IC3 report and get the case number, then try appealing the outcome. Speak with a person, never rely on their automated system.
Be very clear and detailed. I have won following this method.
