Buying used logic board that was in a MacBook most likely damaged on the outside (screen cracked, trackpad etc) is fine if its in perfect condition, its crazy talk if you don't know your buying a b stock - refurbished and you paid a market price...
Here in the United States, if the computer is shrink wrapped and sold in an Apple Store, it is a NEW computer, and has never been handled by anyone else rather than who manufactured it. I don't know what wack laws you have where you live, but consumer protection laws have been around for the last couple of decades in most of the developed world.
Now I will also say this: If you buy a different logic board, and put it inside of your computer, not only will that create a serial number mismatch, but your board may not work at all due to certain components needing to be paired to it during manufacture. Only Apple Authorized Service Providers have these pairing tools.
Replacing a logic board on any laptop computer that has been sold in the last five years, apple or not, is very difficult and non-successful in most attempts. At least Apple can service it and give you a 90 day warranty.
The Macbook 12" has been known in certain circles to be one of the worst computers apple has ever created. Whether or not yours was made in a different factory most likely has nothing to do with it, one way or another every one of these computers is going to have a problem, big or small. I'd put down the caffeinated conspiracy latte you've been drinking.
