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Why does a stock price drop as soon an I purchase several thousand shares at market price?
You might consider learning how the "matching" or "pairing" system in the market operates.  The actual exchange only happens when both a buyer and a seller overlap their respect quotes.   Sometimes orders "go to market" for a particular volume.  Eg get me 10,000 Microsoft shares now.  which means that the price starts at the current lowest seller, and works up the price list until the volume is met.  Like all market it trades, it has it's advantages, and it's dangers.  If you are confident Microsoft is going to bull, you want those shares now, confident you'll recoup the cost.  Where if you put in a priced order, you might get only none or some shares. Same as when you sell.  If you see the price (which is the price of the last completed "successful" trade.  and think "I'm going to sell 1000 shares". then you give the order to the market (or broker), and then the same as what happened as before.  the highest bidder gets as much as they asked for, if there's still shares left over, they go to the next bidder, and so on down the price... and the last completed "successful" trade is when your last sale is made at the lowest price of your batch. If you're selling, and selling 100,000 shares. And the highest bidder wants 1,000,000 shares you'll only see the price drop to that guys bid.  Why will it drop (off the quoted price?).  Because the quoted price is the LAST sale, clearly if there's someone still with an open bid on the market...then either he wants more shares than were available (the price stays same), or his bid wasn't as high as the last bid (so when you sale goes through, it will be at the price he's offering).   Which is why being able to see the price queues is important on large traders. It is also why it can be important put stops and limits on your trades, een through you can still get gapped if you're unlucky.  However putting prices ("Open Orders" vs "(at)Market Orders") can mean that you're sitting there waiting for a bounce/spike while the action is all going on without you). safer but not as much gain (maybe ;) ) that's the excitement of the market, for every option there's advantages...and risks... (eg missing out) There are also issues with stock movement, shadowing, and stop hunting, which can influence the price. But the stuff in the long paragraphs is the technical reasons.