I'm amazed at the stuff Facebook allows in ads. Express political or social heresy, and they'll be right on you. Sell MS-64 commemorative half dollars for $19.95 - no problem. This latest one takes the cake. 1876-CC $20 gold piece, VF-30, on sale for $9.99. Buy seven and get three free and free shipping. What a deal. This must be illegal. Here's a photo.
The coin looks legit. I would buy it. Could be a scorned exwife. Who knows. https://www.ngccoin.com/certlookup/5939641-011/30/
I think it is illegal to sell counterfeits unless the coin has "copy" stamped into it or there is another way to clearly identify it as not genuine. So, are you saying the Facebook ad nowhere identifies it as a copy? Plus, it is a copyright infringement for someone to copy the NGC slab and their trademarks and also various kinds of fraud to pass this off as a genuine NGC product. But with this abomination is coming out of China or the like, there is little even Facebook can do except take it down. And like the Whack-a-Mole, it will just pop up later as someone else. Maybe you could bring it to the attention of the Anti-Counterfeiting Task Force?
The OP says the Facebook ad offers "Buy seven and get three free." I think it is unlikely that there are that many genuine 1876-CCs in NGC holders out there selling for $9.99 each. Much more likely the seller has just downloaded the genuine NGC photo and is using it illegally to push something fraudulent.
Without a link to the actual site, it's hard to tell what is going on. Most likely a bait and switch operation, but I've been known to be wrong in the past. Does it say you are buying the coin pictured? Is there fine print about "you may receive a different coin"? All clues that things are no right.
For those interested, here's a link to this coin and to similar offerings. These folks want $39.95. They always have the same ad format, just a different company name. Pretty amazing. https://www.airamalls.com/collections/gold?&limit=16&sort_by=manual&tags=
The coin is bogus and the holder must be bogus. If you check the NGC verification site, there is a picture of the coin in that holder, and it's not the same. This is a scam, plain and simple. Here is a genuine 1876-CC double eagle, not mine. I handed this when I was a dealer.
If you live in the US it is illegal. If the seller is NOT in the US it isn't. And if the seller is outside the US and you buy it and have it shipped into the US, then YOU are the one who have committed a crime not the seller.