If coin reach very high price, can I always sell it?
It is not guaranteed. It depends on how much 'buy' volume is there (how many people are ready to buy that coin at that price).
For example: A coin reaches $5 price. You want to sell your coins at this price and you put a sell order. Let's assume there are 120 other people who have placed a sell order before you at same price. So if someone is buying at $5, the coins of those 120 people will be sold first. And then it will be your turn. If someone stops buying before your turn arrives, your coins will remain unsold. So it's a game of how much 'buy volume' is there.