MacBook Air wont turn on

Sometimes my mac chooses to not turn on, i do the pram and other resets then take the battery out then it normally works.
Why does this happen i want to sell my mac because im upgrading to a pro but i cant sell it if it sometimes doesnt turn on lol,
anyone have any ideas how to fix it , im currently on 1.6.5 but im upgrading as we speak
THANKS!!

I do that, what I ment is I have to do the resets otherwise I can't turn on my Mac?