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First of all, you would have to be talking about a pretty low-cap stock to be forcing price movements by just a “small quantity” of shares. These are generally penny stocks, which people understand are subject to more volitile movements and even manipulation. Investors understand there is more risk there.
Second, you’re not defrauding anyone by simply buying and selling shares (participating in voluntary transactions), so I see no issues with this from a libertarian perspective.