Be aware...since these cancelled stamps and first day covers are commanding such high prices...its a matter if time they will be forged. Its an age odl dillema in philately and classic example was
Sperati. https://en.m.wikipedia.org/wiki/Jean_de_Sperati

    I have used in the past the American Philatelic Society and the Philatelic Foundation to have rare stamps certified. And trust me...fakes were found that looked as real as the genuine stamp or genuine cancel....or reperforated...or repaired.

   Always reach out to them first prior to sending anything.

   http://www.philatelicfoundation.org/
   https://stamps.org/

    Also hodl on to your receipts.  That might help with provenance...but not 100% either