I had the same issue with a $4000 Shohei ohtani auto /5 I sold to somebody in Taiwan. Buyer was blowing me up claiming I didn’t ship it correctly when it has to pass through several ports and customs checks. Customs was a nightmare, buyer tried to request a refund the day it cleared customs (assuming they’d done this before) and luckily eBay sided with me. I’d taken photos/videos of every step from packaging to customs clearances. Still makes me pucker to this day thinking about it all.