how do they transfere the BTCs when u buy or sell face to face in cash?
In my experience, they tap a button on their phone's browser. A moment later you get the secret "success" code that localbitcoins gave you beforehand, txted to your phone. So you know for sure that they did the BTC transfer. (I'm of the understanding that they can also txt a code to localbitcoins if they don't have a browser-enabled phone.)
Because they pre-escrowed it to LocalBitcoins, you also don't have to worry about someone attempting to double-spend, though how you'd do that in-person in practice escapes me.