Is it illegal to sell alien flag stickers?

Illegal. According to Article 18 of the National Flag Law of People's Republic of China (PRC), it is very clear that the national flag and its design shall not be used as trademarks and advertisements, and shall not be used for private funeral activities. The main body of the national flag consists of two triangles, symbolizing the peaks of the Himalayas. The red background of the national flag is the national color of Nepal.