Which industry is good for Shenzhen just to do sales?

Personally, I suggest that you still engage in your numerical control industry, because you are studying this major and your professional skills will be relatively stronger. You can choose to sell machine tools or knives, and you can choose Japanese-funded or European-American enterprises as the selected enterprises, because the treatment of these companies is relatively better, such as Shanshan in Shenzhen, sandvik Lehman Shenzhen Office, Sumitomo Shanghai Office in Shenzhen and three in Shenzhen. All these are ok, but most of them will have some English requirements.

Well, because I have worked in this line for ten years myself, I know this line very well.

You can search the recruitment situation of these companies online.