pepactonius : indeed, they did that already - numerous modern fakes are in plastic tombs, and contemporary ones.
The stuff is, serious auction houses will give you your money back if there is an issue for raw coins (and for certified ones, NGC will).
I don't think we was fake slabs for such coins yet (so if there is an issue, the grader warranty will apply)
In any case, you are sure that the coins were at least checked, that you have proper pictures on hand.
ebay is risky as you are buying from un unknown 3rd party most of the time (although you still have some very serious sellers there though .).
In any case, you have to really know well those coins when buying, even more when it's from unsafe sources as
ebay.
At least, the seller above accept return avec 14 days :)