Is it permissible for the husband to increase his income by buying and selling used items, such as buying a sofa that its seller does not need and selling it for a higher price?

There is no harm for your husband in selling used items for a price higher than he bought them for. He must inform the buyer that they are used if the buyer is unaware of that, because the Prophet, peace and blessings be upon him, said: "The two parties to a transaction have the option as long as they have not separated. If they are truthful and make clear, their transaction will be blessed. But if they lie and conceal, the blessing of their transaction will be effaced."