Cigarettes bought online are safe to the degree that smoking any kind of cigarette is safe. However, e-cigarettes bought online from China do present additional risks to the already harmful habit.
As of 2015, the CDC considers cigarette smoking harmful to the human body, and notes that 480,000 Americans die every year from the long-term effects of cigarette smoke. Therefore, smoking cigarettes from any vendor, online or otherwise, is not a safe activity. Almost every major brand worldwide is available online, no different in danger from the counterparts available in brick-and-mortar stores.
However, some countries' cigarettes do have harmful features not found in domestic brands. For example, e-cigarettes from China have much higher levels of hazardous nickel and chromium vapor. If buying cigarettes online, you should avoid foreign brands from countries with loose manufacturing regulations.