Am I obligated to inform customers that the pipes used are 2.8 mm thick instead of 3 mm, knowing that I sell at the price of the lesser thickness?

It is not permissible to sell pipes claiming they have a certain thickness when they are actually less than that, even if sold at their real price. This is because the thickness of pipes is something the buyer desires, and deception in this regard deprives the buyer of a significant benefit. It is not permissible for the seller to deprive the buyer of a legitimate purpose of purchase except with clear disclosure; otherwise, the seller has cheated and engaged in forbidden deception. This is based on the Prophet's (peace be upon him) saying: "Whoever cheats is not one of us," and his saying: "The two parties to a transaction have the option [to annul it] as long as they have not separated. 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 eradicated."