Can You Counterfeit Annual US Mint Proof Sets?

It has been done for some time or at least a variation of it.

It really started back in 1999 when they stopped sealing the proof sets. For awhile on ebay there was a big market in "error" proof sets that were missing one of the State Quarters and had two of one of the others. The scammers would buy five normal sets, open them, swap the quarters around and sell the "error" sets for $300 to $400 apiece.

Then in 2000 some special "VIP" 10 coin 1999 proof sets began hitting the market. The 1999 sets were nine coin sets because the SBA came out late in the year and wasn't included in the set. The sellers were promoting these sets as special sets made up for some VIP's and they were very rare. The scammers were opening up a 10 coin 2000 proof set and replacing the coins with 1999 coins and the 1999 SBA. Then putting them in a 1999 box. These sets were bringing as much as $1500. If you knew your proof sets you knew what was going on because they changed the color of the insert between the 1999 and 2000 sets.

Then the price of the 1999 silver proof sets went through the roof scammers opened them up slabbed the silver coins and sold the slabbed silver set, then refilled the set with clads and put them back in the silver proof set box and sold them on ebay.

I have also seen 1990 prestige proof sets that have been opened and the proof cent replaced with a nice BU Philadelphia coin, reassembled and sold as the rare 1990 missing S set. They liked using the Prestige set because the outer holder hid the signs of tampering on the proof set holder inside. These would bring a couple grand.

It would be pretty easy to add some sort of seal or sticker to the edges of the holder to make the sets tamper-evident. Stupid mint.

I do not agree.

The proof set packaging is only meant to temporarily hold coins until they are opened up to fill albums.

As noted above.
The only way to make sure it has not been tamered with in one way or other is if you go the Mint and buy one as it comes off the line. Of course that doesn't mean a Mint worker is not a counterfeiter though. Would be a great job and place to pass of your own fakes.

Any circulation strike good enuf to be confused with a proof would be worth more than the proof.

An important exception in modern proofs - 1999 Susan B. Anthony dollars were minted in Philadelphia and sport a "P" mintmark.

Please tell me how.

Regarding the 1973 ~ 1979 OGP lenses, I had the opportunity to look at mine this weekend.

The two halves were sealed together around each coin, but not along the outside edges. However, judging by their current appearance, they came apart relatively cleanly.

When put back together, it is difficult to see that they had been taken apart. The only hint of them being tampered with was along the inside edge around each coin hole. If coins were put back in place, this evidence would not be visible.