If you place a minimum bid on an item then it will not sell for less than the minimum bid. If nobody paid the buyout price, the item will automatically go to the highest bidder for a price 5% higher than the 2nd highest bidder (or the minimum bid if he's the only bidder). If (and only if!) nobody paid the buyout price and nobody bid on your item, the item will be returned to you.
Don't place a minimum bid on your item that is lower than what you are willing to sell the item for. For example, if you would rather keep the item than sell it for less than 1 million, don't list minimum bid of less than 1 million. Simple as that. But yes, you must decide in advance. Once you list an item there is nothing you can do.