In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
The jeweler isn’t responsible for investigating why a person wants to buy items of jewelry since, in principle, selling golden adornments to men is permissible.
Answering your question, the General Iftaa’ Department in Jordan, states the following:
Sale is permissible, and Muslim scholars have stated that every sale transaction should be dealt with according to what is apparent when it is hard to investigate the intention of the buyer.
Al-Imam Ash-Shafi`i said, “Judgments should be based on what is apparent because what is hidden is known only by Allah, The Almighty; therefore, one who judges people according to conjecture violates the injunctions of Allah and His Messenger since Allah is the One Who determines the reward and the punishment for the Unseen. He, the Almighty, has commanded his servants to deal with each other based on what is apparent, not what is hidden.” (Al-Umm,4/120)
In conclusion, the jeweler isn’t responsible for investigating why a person wants to buy items of jewelry since, in principle, selling golden adornments to men is permissible. However, if he knew for sure that the buyer intends to use them unlawfully, then he isn’t permitted to sell them to him because doing so is helping in sin and rancor.
Almighty Allah knows best.