Exactly, in after-hours trading there must be a seller for you to be a buyer. That's how it works in all trading, actually, it's just that during the normal trading day there are so many more exchanges happening it's easier to find that buyer.
So let's use this as an example:
Stock XYZ closes the regular trading day at $10/shares. That night, Cramer speaks highly of XYZ and you go to your online brokerage hoping to buy 1,000 shares at the $10/share it closed at. For that to happen there must be someone also trading after hours willing to sell that number of shares at the price you specified -- there's no such thing as a market order AH. But because Cramer mentioned the stock, they're not likely to want to sell their shares at that cost.
It helps to think of post- and pre-market trading as a sort of peer-to-peer thing. A small number of traders trying to make deals among themselves.
Stocks gapping up (or down) are one of the most misunderstood things in trading. It makes sense to think that a stock opens a trading day at the price it closed the previous day, but that's not how it works. Before each day, a market maker (a specialist for the stock exchange in charge of setting prices at opening and other certain situations) determines what they believe the opening price should be, based on the previous day's closing and changes in demand that have occured overnight.
So, to use the same example, XYZ closed at $10, then Cramer said on his show it was a great stock. There was a rash of people buying the stock in after-hours, exchanging shares as high as, let's say, $12/share.
The next morning, instead of opening at either $10/share or $12/share, the market maker may have set the price at $11.50/share. So the gap up happened instantly. Now, if you bought at $12/share in AH, you've got a paper loss to start the day. Maybe the stock will ramp up during the day, but maybe not. A lot of people who held the stock already will be selling to grab their own profit.
Trying to figure a way to jump on the Cramer picks before everyone else is close to impossible. Keep in mind that thousands of people watch his show and are trying to do the exact same thing. It's a lot easier to overpay for a stock he mentions than in is to find a way to get a great price and make a quick profit. That's why he's always telling viewers to be patient in their buying -- determine what a fair price for a stock is, and not pay more than that; don't jump into an AH trade in haste.
In stock trading and in gambling, discipline results in more profit. Rushing into an AH trade just to grab a stock immediately is like making a bet late at night without much thought, trying to make up for a day when well-thought-out bets went against you. Sometimes it works, but more often that not it just results in an even bigger loss.
