I just checked on 4 of mine and they all sink. Many accessories are supposed to sink. I sell vintage Star Wars items on eBay every week and this sounds like an uninformed buyer (I’m seeing lots of new “collectors” recently who don’t seem to know what they’re bidding on and ask basic questions) or a buyer trying to get a free discount. Tell him to file a return, check the item you get back to make sure it’s the exact items you sent him, report him to eBay if needed, refund him, and block him. Then re-sell the item.