Hi! What’s the correct form to say: the product is (on / in) the seller hands? I will appreciate your help! :?
If you mean “he/she is responsible for what happens to, or has control of, the product” (metaphorical expressions “it is in your hands/out out of my hands), then “in” is the fixed form”. If you want to speak literally you would say “on” when the product is worn on the hands or is applied to the hands, as with a cream, for example. If the person is holding the thing on a horizontally flat ,or almost flat, palm, you could also use “on”. If the person is cupping or enclosing the product with his/her hands, you would use “in”.
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And if you say that ‘the seller has product X on hand’, that means that the seller has product X in stock.
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Not sure what that’s got to do with “is in/on the seller’s hands”.
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That was just my experience with ESL students and their questions kicking in, Molly.
However, feel free to look at my post simply as “bonus information”. 
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Hi Lin
I’d like mention something else that Molly did not mention:
If you say “the product is on the seller’s hands”, the sense is often negative. That would tend to suggest that the seller is unhappy about the large quantity he/she has in stock (too much) or that the seller is unhappy to have any of the product at all.
If you say that a seller has a product “on hand”, that is neutral and simply means that the seller has the product in stock or available.
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thank you very much!!! 