If Amazon even thinks that your selling counterfeit items they close you down in a jiffy and it can be months until you get back your account ( if ever ). It's in their T&C.
Depends on the item but as a seller on Amazon:
1. Most people have no clue if it's fake or not unless it's really off, obviously every case is different.
2. It's actually pretty easy if you know what you're doing to get back on and continue selling even if you're selling counterfeit items.
Obviously when an item comes from China there's a big reason for concern, however counterfeit coming out of Amazon warehouses also exists.