What I am worried about is, if I hold huge amount of worth of coins and I want to sell it but no one wants to buy that(i.e no BUY orders) so what will happen?? Will my coins just be sitting there? or will automatically gets sold as market order?
First, that is unlikely to happen unless you have a lot of coins or you are on a very small exchange. For example, on the Bittrex exchange right now, the first buy order in the book is to buy 29 million DOGE for around 11 BTC. Your 2 BTC worth of DOGE will easily be filled.
If you place a market sell order for more bitcoins than there are in the order book, then you will clear out the order book. I don't know what will happen to the unfilled portion of your order. It will probably be canceled.
Note that if you clear out the order book, you will have sold most of your coins at rock-bottom prices. You don't want that to happen, of course. If you have a lot to sell on a small exchange, you must sell slowly in order to avoid selling for very low prices.
Thankss I think now I understand it more clearly..
about the last part you said... selling most of my coins at rock bottom prices, how can it happen since when putting sell order I've put a price?? or does it just tries to fulfill the order.
What my plan was to trade ONE coin only with my whole BTC account balance each day... but now it seems its not completely doable.