I have an item that wasnt manufactured by the original brand but its the same exact item. Is it ok to sell it under the original brand name? No customer would be able to notice any difference in the product its just a brand that they are more familiar with.

No, that would be deception (Gneivat Da'at) which is clearly prohibited. You can publicize something which is true but gets the message over, e.g.:"Identical with the original, just less expensive".