Shenzhen, near Guangzhou, is famous for consumer electronics. 
-Flights from Shenzhen to Yiwu:
According to one of the largest air tickets agent in China(ctrip.com), there is one flight on Sundays, Mondays, Wednesdays and Fridays.
Leaves Shenzhen Baoan Airport at 14:10, Arrive at Yiwu Airport at 17:05, stop by at Xiamen
*No direct flights on Tuesdays and Thursdays.
Full price is 1,100 RMB. If you book 2 weeks in advance, you can usually get a 50% off. There will be 50 RMB more for airport fees and fuel taxes.

-Trains From Shenzhen to Yiwu:
T212 leaves Shenzhen Train Station at 13:28, arrives at Yiwu Train Station at 04:07 in next early morning. 14 hours and 51 minutes.
K1210 leaves Shenzhen Train Station at 10:09, arrives at Yiwu Train Station at 06:50 in next early morning. 20 hours and 41 minutes.

-Buses from Shenzhen to Yiwu:
There are up to 10 buses leave from different bus stations of Shenzhen for Yiwu each day.
It can take up to 15, 16, or even more hours by bus. Price is around RMB 300.
Bus would be the third choice after plane and train.