Not true. There may be no one to buy at or sell at the recent prices. And, there may not be offers/bids in the volume an institution needs. But, there are always offers to buy or sell at prices away from recent prices. There's a stack of them on both sides. I have "level 2" visibility of these orders. They just sit there waiting for folks who make "market orders", i.e. buy or sell at the best available price. If nothing else, someone will offer to buy 100,000 shares of IBM at a penny a piece or sell them to you at $1000 a share.

I always use limit orders so I know the max I will pay or the min I will get. Heck, I often pick a price that's inside the current best bid offer spread. Sometimes those orders just expire at the end of the day, but mostly they do get executed.