When I'm not sure about the seller, I always send a convo and ask some questions. If she/he responds quickly and in a professional manner, I will buy from them. If they never respond or write as if texting back to a friend, I won't probably buy from that seller. I don't mean being friendly is bad, but I am talking about being rude (no "thank you" or hi,) not spelling out words (ie, u for you etc.,) or doesn't know how to write sentence.
I understand there are many non English native sellers on Etsy (and I am one of them) but you can tell the difference when people make mistakes because they are not native or because they don't care. So far, all the sellers I bought from responded quickly and very professional manner, and I never had any problem with them.