General:
* Contrabandist
* Bootlegger
* Runner
* Trafficker
* Dealer
* Fence
* Black marketeer
More specific:
* Drug smuggler: Narcotics trafficker
* Arms smuggler: Weapons dealer
* Human smuggler: People trafficker
* Art smuggler: Cultural property thief
Informal:
* Smuggler
* Hoodlum
* Criminal
* Outlaw
The best synonym to use will depend on the specific context of your writing. For example, "contrabandist" might be more appropriate than "smuggler" if you are writing about someone who is smuggling goods into a country illegally, while "trafficker" might be more appropriate if you are writing about someone who is smuggling drugs or people.