What is the ruling on selling used phones after maintaining them or buying them from merchants without clarifying that to the customer?

There is no harm in selling used goods. However, when selling them, one must be truthful in describing them and stating their condition, as the Messenger of Allah (peace be upon him) said: "The two parties to a transaction have the choice as long as they have not parted. If they are truthful and clarify, they will be blessed in their transaction. But if they lie and conceal, the blessing of their transaction will be effaced." Therefore, it is not permissible to describe an item's use as light unless the seller actually knows that to be the case.