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Can I copy & use someone else’s product description to sell my item on my online store?
Never do that. Duplicate content penalty is more fatal than a cannibalized page. In the case of duplicate content you get penalized while leaving your product pages without content will just keep your pages out of the search index.
It is better to not rank than to get your pages go back in SERP. I say this because when search engines slap a penalty it affects the complete site, not just those pages with the problem. This is not the case for cannibalization.
If you are looking for a quick and easy solution to generate unique product descriptions check for it here.