Is this a gramatically correct statement? - 1
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"I'm not in possession of the lens you mentioned, but it looks like it is compatible with the Nikon D3100 camera."
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Better would be: "I don't personally own the lens you mentioned, but it looks as though it would be compatible with the Nikon D3100 camera."
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Yes!
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The only fault is your use of "like" with a verb. I almost hesitate to point it out because it's so widely used, I'm sure it'll be standard English within a generation: but strictly speaking you should say… "it looks AS IF (or AS THOUGH, as Texan said) it is compatible"…
In other words, you can say, "It looks like a (noun)" but you shouldn't say "it looks like it (verb)".
Nothing wrong with "I'm not in possession of the lens… "
I believe the situation is a shop owner talking about what he has in stock, rather than what he personally owns. But in either case, your phrasing is perfectly OK.