Is working in a factory that sells finished products as if they were locally assembled, while deceiving customers into believing that, permissible (halal)? And is the salary earned from this work permissible (halal)?

Placing the factory's logo on products not manufactured by it, to deceive customers into believing they are factory-made, is considered forbidden deception (ghish) and invalid fraud, even if the products are of good quality. This is based on the Prophet's (peace be upon him) sayings: "Whoever cheats us is not of us," and "The two parties in a transaction have the option (to cancel it) as long as they have not parted. If they are truthful and clarify, blessings will be bestowed upon their transaction. But if they lie and conceal, the blessing of their transaction will be erased."

Therefore, cheating and deception in selling by changing a product's name or describing it with qualities it does not possess to mislead the buyer is impermissible. It is obligatory to cease this forbidden act and to disclose the truth to people.

As for your work in the factory, which is limited to verifying technical specifications before purchase, it is permissible, and there is no harm in it. The salary you receive for it is lawful.