What's always held by Sotheby's? (10)
I believe the answer is:
apostrophe
I'm afraid I can't explain this answer.
Can you help me to learn more?
(Other definitions for apostrophe that I've seen before include "At which point discarded letters" , "Sign of omission" , "Often misplaced punctuation mark" , "address?" , "in Charley's Aunt ?" .)