I bought a bunch of sealed envelope 1962-64 proof sets many years ago because the price was just about the price of the silver in them. They were original Mint envelopes in great condition.
I decided to do an experiment and steamed open one of the 64's, took a peek at the coins (nice coins)and then just resealed the envelope. Very easy and impossible to tell it was opened.
Buy them for the coins and do not pay a premium because they are "unopened"
As others have said an ebay trick is to open the envelope, remove the set and replace with cheap 70's and 80's coins of the same weight. I would open envelopes and verify correct coins are inside. I would not cut open the mylar packing though.
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