> Don't online drug marketplaces lead to reduced gang activity?

Online isn't the important factor here.

> One does not need a gang and violence to sell drugs online.

Gangs and violence aren't there to support a marketplace. They don't help you find customers or customers to find you. They don't improve the efficiency of exchange. They're there to enforce outcomes. Selling drugs leads to outcomes that don't care whether the buyer and seller found themselves online.