This is Matsya avatar of Vishnu. The identification is based on the four-armed deity with the lower half as fish. The rear upper arms hold two of Vishnu’s attributes Shankha and Lotus (most likely).
Vishnu took this Matsya avatar to rescue the Vedas and nuclei of the universe from possible destruction in the deluge. Another version of mythology is Vishnu took this avatar, to rescue Veda’s from Hayagriva Asura. The Vedas were taken away from Brahma and hidden in the ocean. Hayagriva Asura is different to Hayagirva Avatar, a minor Avatar of Vishnu, and more about Hayagriva Avatar in another blog.
In some of the earlier sculptures, Matsya avatar is shown as fish, theriomorphic form.
As per J N Banerjea, Matsya avatar was originally associated with Brahma. The attribution of this avatar to Vishnu is a reflection of the strengthening of Vaishnavism.
The dealer sold it as a piece from Nepal. The Vanamala, the flower garland falling to calf level, suggests this idol is from North India.
This bronze is just over 5 cm in height. This is a molded piece. I do not have any guess as to the age. The bronze shows signs of handling.
Temples for Matsya are rare, just a handful. It is rarer to see bronzes of the size meant for home worship. It is for that reason Matsya is included here.