I never deal with traders i only deal with Factory, but i am UK person living in Shanghai.
Traders maybe have good english knowledge in many cases i find in Hong Kong they have much experience because HK is full of foreginers. Mainland China very little experience even in Shanghai i feel more than other places still limited.
What you need to do from initial contact need to build up trust as now so many scammers about buyer very frightened. If English is bad they then maybe worry more you maybe do not understand their order and make error and they lose money so buyer will usually deal with good english speak but pay little more for make them feel better but still can be scammed.