Obtaining a tourist visa for China in Seoul for a non-resident

I'm afraid you are not able to get the visa in Seoul, since non-Korean applicants in South Korea must provide the alien registration card in South Korea with at least 6 months remaining validity. You'd better apply for a China visa in your country before the trip. A China visa is usually issued with 3 months validity, I don't think it will expire when you enter Shanghai if you won't stay in Seoul for a long time.