[bonestamp2]

The market price is (basically) the highest price that someone else is bidding to buy at, so your offer to sell at that price guarantees there is a buyer because the buyer's bid for stock at that price is already there when you place your offer to sell.

It can get a little more complicated than that though -- you might be trying to sell 1000 shares and the highest bid might only be for a quantity of 500 so the "market" price for your first 500 shares will be different than the next 500 (unless there are other bids at that same price, which there often are but there's no guarantee on the volume you'll be able to sell at that price).

And that's why they call it a stock exchange... it's just a bunch of people (and companies) making bids and offers to "exchange" stocks at difference prices. When a buyer and a sell agree on a price, you have a transaction (trade). Your offer to sell at market price is just an agreement between you and someone else willing to buy at that price.