Worse case scenario, what could happen if you get a fake key? Stops working after a period of time or could a virus be possible from something like this? Why does ebay or other sites allow the sale of fake keys? Takes too much resources to weed out the bad from the good?
Since you're just buying the key, it would be up to you to download it from a reputable site (ISO from Microsoft). Some sellers sell the key along with a generic USB with an installer, but that's a bad idea to do it that way. If the key isn't legit? Microsoft periodically updates its database to deactivate keys that have been sold / used against their service terms. So they could flag the key, and then you're out of what you paid for it.
As far as why Ebay and other sites allow it? Hard to say for sure. The most likely reason is it's too much for them to monitor, along with having sellers in different countries with different laws, they likely let Microsoft deal with it.