An arms dealer is a merchant who manufactures and/or sells weapons, military technology and equipment. This was typically done illegally or on the black market.
The Minosians were known as dealers of advanced weaponry. They supplied arms to the combatants of the Erselrope Wars. Unfortunately their advanced weaponry, such as the Echo Papa 607, ended up destroying them. (TNG: "The Arsenal of Freedom")
Bajoran Latha Mabrin was a smuggler and an arms dealer before he found the Prophets and became a Vedek. (DS9: "The Darkness and the Light")
According to Enabran Tain, Dukat was once involved with arms merchants. (DS9: "The Die is Cast")
Gaila, a cousin of Quark, was employed as an arms dealer. He worked for fellow arms dealer Hagath. He briefly recruited Quark into his business, hoping that he could retire to his own moon. Quark, however, had a change of heart and decided not to remain in the business.
The Bajorans bought many of their weapons to fight the Cardassians from arms dealers, including Hagath. After the Cardassians left, the Bajorans were willing to tolerate further activity from those dealers within their territory because, as Kira Nerys said, they would not have won without people like him. (DS9: "Business as Usual")