بسم الله الرحمن الرحيم
Pig is impure by nature. Its hair is also impure. It is not lawful to sell or buy the brush made of its hair. So it is mentioned in Fatawa Mahmoodiya etc regarding such brushes.
Note: If you are sure that the said brush is made of pig hair then the ruling shall be same as mentioned above. (Z)
Allah knows Best!
Darul Uloom Deoband