I think it is more convenient from Guanzhou to Yiwu. The train from Guangzhou to Yiwu is about CNY327 for hard sleeper and CNY499 for soft sleeper of single trip. Price of roundtrip is twice of the single trip. Well, you can look for more information through Guangzhou Train Schedule(https://www.travelchinaguide.com/china-trains/guangzhou-schedule.htm) .

i suggest take a flight from Guangzhou,much quicker.