It's entirely possible that a "basement slabbing company" would slab a fake coin, either accidentally or deliberately. There can certainly be no guarantee the coin is genuine. And you'd kind of need to bust the coin out of the slab in order to test it properly for authenticity.
EBay isn't punishing you for trying to sell a fake coin; it's punishing you for trying to sell a "fake slab". A slab is supposed to convey some aura of legitimacy, and basement slabs do not deserve that legitimacy.
As a result, eBay has not outright banned the sale of basement-slabbed coins, but they have put strict rules in place that apply only to coins.They include that you cannot mention the Sheldon-scale grade the basement slabber has given it (for example, "MS-65"), or use then name of the basement slabber "INB" anywhere in the item description - you need to treat it as if it were a "raw" unslabbed coin, and give only an opinion of an adjectival grade (for example, "Very Fine").
It is, of course, a trap for those who are not regular coin buyers or sellers, and thus may not be aware of these policies. Sorry about that. But, now you know.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis