Do you have a Proof of Delivery? How long did they take to arrive? How long did it take to report an issue?
Only, if the forks took a week to get there, I’d be more inclined to think “the postman did it” than if they arrived the next day; ditto if it took the recipient ten minutes to go “hey, faulty” and kick them back.
Occam’s Razor though, innit. Let’s say the courier was the culprit. What logic is there in opening a package, stripping a fork, replacing the internals with matching broken ones, packaging it all back together, and then completing the delivery?
Who is more likely to have broken parts lying around which match your fork; a shop which buys and sells used parts or Postman Pat?
You shipped them, freshly serviced and full of oil. They received them in an allegedly uncompromised box, devoid of oil and broken. Based on evidence presented here it’s my considered opinion that (much like your forks) one of you is full of shit. Assuming it’s not you and you didn’t send them via David Blaine Courier Services, the guy who you sold them to could be innocent but if so then they were knobbled whilst in his care.