I will be using LocalBitcoins, if I have 0.5 BTC I can sell up to 85% of that in my own advert then the rest on another person's advert. Is there a way to make a profit this way? Or is 0.5 simply not enough to start with?
Yep. That is pretty big to start trading but just put in to your mind that you can loss in trading luck and skill can make you a large profit. I will suggest that you can deposit 0.001 first to know the system of trading and how it will work. Just for one week know all about trading and then deposit the 0.5 bitcoin to formally start your trading. I hope you will make q large profit from it 
happy trading!Starting from small amount while you are still learning will be good. Actually there is no mandatory minimum amount, whatever you have you can use, but properly. BTC 0.50 is quite enough if you have that much only, and personally its a large amount for me.