The Skiller can decide whether he wants to give up to 5 minutes free. Its good in order to check if they match. And in case that the 5 minutes free was enough for the whole service the user can tip the Skiller and everyone will be happy

How do you prevent people from abusing this 5 minute grace period?
We don't. its up to the Skiller to give them. If the Skiller notice that the user is not serious he can end the call immediately.
I see. Is the call charged per second or?
It's probably paid per second yeah.
I hope there will be option for some fixed amount too,so Skillers dont drag the call just to get more money.
Well,in that case you can always rate him lower if you see he is doing that.