There are several reasons.
First, if you sell your stock "at any price", you may be selling it for less than you originally bought it for. Thus you will take a loss right at the beginning of your scheme. If you "rinse and repeat", the problem only gets worse. Every time you sell your stock, you will have to sell it at an even lower price in order to lower the price even more. Then you buy it back and. . . just resell it an even lower price? It should be clear that you are not making any money this way.
Second, even if you don't sell it at an absolute loss, you must sell it at a relative loss in order to lower the price. In other words, if someone will currently buy your shares for $X, and you want to lower the price, you must sell them for less than $X. But you could have made more money by selling them for $X, since someone was already willing to buy them at that price. In order to bring the price down significantly, you have to sell the stock for less than people currently believe it is worth, which means you're incurring a loss relative to just selling it at the market rate. Of course, you can still make money if it goes back up again, but selling it at an extra loss this way just makes it harder to break even.
Third, if you sell the stock at $X, whoever you sold it to is not going to sell it right back to you at $X, because then they would not make any money. You could in theory buy it from someone else, but the same principle holds: if the stock price has just gone down, people who have it may be waiting for it to go back up. This is doubly true if anyone suspects you have been trying to manipulate the stock price, because they will then suspect that the price drop is artificial and it will soon go back up.
Fourth, even if someone did sell it right back to you at the price you sold it for, then what? You now hold the stock at a lower price, but you don't gain unless it goes back up. If it wasn't going up before until you took action, there is no reason to suppose it will go back up now. In fact, if you had enough shares to significantly influence the price, other people may have been fooled into thinking the value is actually lower now.
The basic problem is that, in order for you to buy it at a low price, someone else has to sell it at that low price. It is easy to sell someone a stock for less than it's worth, but it will be hard to get people to sell it back to you for less than it's worth. If you engage in deceptive practices to get people to do this, you may be guilty of securities fraud.