Pros: Might look cool, is cheap.
Cons: Won't be very durable, the app for it will be shitty and closed-source(probably malware), the battery will probably suck, any vital sensors won't be accurate, the screen will probably suck, and it will probably have a bad UI.
In summary: Save up for a better watch.
The best advice is just to be cautious. Obviously don't send money before you get something and make sure it's functional. Facebook marketplace is a good place to find deals if you know to be safe.