An arms dealer, arms merchant, weapons dealer, or weapons merchant was a merchant who manufactured and/or sold weapons, military technology, and equipment. This was typically done illegally or on the black market.
While on a covert mission to Qo'noS in 2257, the mirror universe Philippa Georgiou sold a pair of Nausicaan disruptor pistols to an Orion arms dealer for two thousand darseks. (DIS: "Will You Take My Hand?")
The Baron Grimes was an arms dealer, who made considerable profit during the 2250s Federation-Klingon War. (DIS: "Magic to Make the Sanest Man Go Mad")
In 2259 in the alternate reality, a USS Enterprise away team to Qo'noS posed as K'normian arms dealers. (Star Trek Into Darkness)
The Minosians were known as merchants of advanced weaponry. They supplied arms to the combatants of the Ersalrope Wars. Unfortunately, their advanced weaponry, such as the Echo Papa 607, ended up destroying them. (TNG: "The Arsenal of Freedom")
Upon approaching Kriosian Ambassador Briam, the Ferengi Par Lenor said that generally peace was good for trade, unless one happened to be an arms merchant. (TNG: "The Perfect Mate")
This statement echoes the Ferengi Rules of Acquisition 34 and 35.
According to Enabran Tain, Dukat was once involved with arms merchants. (DS9: "The Die is Cast")
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")
The Bajorans bought many of their weapons to fight the Cardassians from arms dealers like 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.
Gaila, a cousin of Quark, was an arms dealer in business together with Hagath. He briefly recruited Quark into his business in 2373, hoping that he could retire to his own moon. Quark, however, had a change of heart and decided not to remain in the business. (DS9: "Business as Usual")
Quark's actions directly led to Gaila's ruin, so that one year later Gaila lamented that he "was a thriving weapons merchant until I went into business with you". (DS9: "The Magnificent Ferengi")