Is it permissible to sell an old radio that was bought at a low price and sell it at a high price as an antique for decoration?

There is no harm in setting the selling price of a commodity at more than double its cost, as long as it is free from deception and fraud. There is no specific limit for profit; rather, it is permissible according to what the two parties agree upon, for the Messenger of Allah, peace and blessings be upon him, said: "Indeed, selling is by mutual consent."